A month is a unit of time, used with calendars, which is approximately as long as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of Moon phases; such months are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth-Sun line, are still the basis of many calendars today, and are used to divide the year.

Types of months in astronomy

The following types of months are mainly of significance in astronomy, most of them first recognized in Babylonian lunar astronomy.
  1. The sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference. It is about 27.32166 days. It is closely equal to the time it takes the Moon to pass twice a "fixed" star.
  2. A synodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase as seen by an observer on Earth. The mean length of the synodic month is 29.53059 days. Due to the eccentricity of the lunar orbit around Earth, the length of a synodic month can vary by up to seven hours.
  3. The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month days, because of precession of the equinoxes.
  4. An anomalistic month is the average time the Moon takes to go from perigee to perigee—the point in the Moon's orbit when it is closest to Earth. An anomalistic month is about 27.55455 days on average.
  5. The draconic month, draconitic month, or nodal month is the period in which the Moon returns to the same node of its orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth's orbit. Its duration is about 27.21222 days on average.
A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. The Sun moves eastward with respect to the stars and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun.
An anomalistic month is longer than a sidereal month because the perigee moves in the same direction as the Moon is orbiting the Earth, one revolution in nine years. Therefore, the Moon takes a little longer to return to perigee than to return to the same star.
A draconic month is shorter than a sidereal month because the nodes move in the opposite direction as the Moon is orbiting the Earth, one revolution in 18.6 years. Therefore, the Moon returns to the same node slightly earlier than it returns to the same star.

Calendrical consequences

At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59 days: a 30-day full month followed by a 29-day hollow month—but this is only roughly accurate, and eventually needs correction by using larger cycles, or the equivalent of leap days. Additionally, the synodic month does not fit easily into the year, which makes accurate, rule-based lunisolar calendars complicated. The most common solution to this problem is the Metonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years. However, a Metonic calendar will drift against the seasons by about one day every 200 years. Metonic calendars include the calendar used in the Antikythera Mechanism about 2,000 years ago, and the Hebrew calendar.
The complexity required in an accurate lunisolar calendar may explain why solar calendars have generally replaced lunar calendars for civil use in most societies.

Months in various calendars

Beginning of the lunar month

The Hellenic calendars, the Hebrew Lunisolar calendar and the Islamic Lunar calendar started the month with the first appearance of the thin crescent of the new moon.
However, the motion of the Moon in its orbit is very complicated and its period is not constant. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore, the beginning and lengths of months defined by observation cannot be accurately predicted.
While some like the Jewish Karaites still rely on actual moon observations, most people use the Gregorian solar calendar.
Pingelapese, a language from Micronesia, also uses a lunar calendar. There are 12 months associated with their calendar. The moon first appears in March, they name this month Kahlek. This system has been used for hundreds of years and throughout many generations. This calendar is cyclical and relies on the position and shape of the moon.

Julian and Gregorian calendars

The Gregorian calendar, like the Julian calendar before it, has twelve months:
1January31 days
2February28 days, 29 in leap years
3March31 days
4April30 days
5May31 days
6June30 days
7July31 days
8August31 days
9September30 days
10October31 days
11November30 days
12December31 days

The mean month length of the Gregorian calendar is 30.436875 days.
Months existing in the Roman calendar in the past include:
The famous mnemonic Thirty days hath September is a common way of teaching the lengths of the months in the English-speaking world.
Any five consecutive months contain 153 days.
The knuckles of the four fingers of one's hand and the spaces between them can be used to remember the lengths of the months. By making a fist, each month will be listed as one proceeds across the hand. All months landing on a knuckle are 31 days long and those landing between them are not. When the knuckle of the index finger is reached, go back to the first knuckle and continue with August. This physical mnemonic has been taught to primary school students for many decades.
This cyclical pattern of month lengths matches the musical keyboard alternation of white and black keys.

Calends, nones, and ides

The ides occur on the thirteenth day in eight of the months, but in March, May, July, and October, they occur on the fifteenth. The nones always occur 8 days before the ides, i.e., on the fifth or the seventh. The calends are always the first day of the month, and before Julius Caesar's reform fell sixteen days after the ides.

Relations between dates, weekdays, and months in the Gregorian calendar

Within a month, the following dates fall on the same weekday:
Some months have the same date/weekday structure.
In a non-leap year:
In a leap year:
The Hebrew calendar has 12 or 13 months.
  1. Nisan, 30 days ניסן
  2. Iyar, 30 days אייר
  3. Sivan, 30 days סיון
  4. Tammuz, 29 days תמוז
  5. Av, 30 days אב
  6. Elul, 29 days אלול
  7. Tishri, 30 days תשרי
  8. Marcheshvan, 29/30 days מַרְחֶשְׁוָן
  9. Kislev, 30/29 days כסלו
  10. Tevet, 29 days טבת
  11. Shevat, 30 days שבט
  12. Adar 1, 30 days, intercalary month אדר א
  13. Adar 2, 29 days אדר ב
Adar 1 is only added 7 times in 19 years. In ordinary years, Adar 2 is simply called Adar.

Islamic calendar

There are also twelve months in the Islamic calendar. They are named as follows:
  1. Muharram محرّم
  2. Safar صفر
  3. Rabī' al-Awwal/Rabi' I ربيع الأول
  4. Rabī' ath-Thānī/Rabi' al-Aakhir/Rabi' II ربيع الآخر أو ربيع الثاني
  5. Jumada al-Awwal/Jumaada I جمادى الأول
  6. Jumada ath-Thānī or Jumādā al-Thānī/Jumādā II جمادى الآخر أو جمادى الثاني
  7. Rajab رجب
  8. Sha'bān شعبان
  9. Ramadān رمضان
  10. Shawwāl شوّال
  11. Dhu al-Qi'dah ذو القعدة
  12. Dhu al-Hijjah ذو الحجة
See Islamic calendar for more information on the Islamic calendar.

Arabic calendar

Hindu calendar

The Hindu calendar has various systems of naming the months. The months in the lunar calendar are:
Sanskrit nameTamil nameTelugu nameNepali name
1Caitra Chitirai Chaithramu Chaitra
2Vaiśākha Vaikasi Vaisaakhamu Baisakh
3Jyeṣṭha Aani Jyeshttamu Jesth
4Ashadha Aadi Aashaadhamu Aasad
5Śrāvaṇa Aavani Sraavanamu Srawan
6Bhadrapada Purratasi Bhaadhrapadamu
7Āśvina Aiypasi Aasveeyujamu Asoj
8Kārtika Kaarthigai Kaarthikamu Kartik
9Mārgaśīrṣa Maargazhi Maargaseershamu Mangsir
10Pauṣa Thai Pushyamu Push
11Māgha Maasi Maaghamu Magh
12Phālguna Panguni Phaalgunamu Falgun

These are also the names used in the Indian national calendar for the newly redefined months. Purushottam Maas or Adhik Maas is an extra month in the Hindu calendar that is inserted to keep the lunar and solar calendars aligned. "Purushottam" is an epithet of Vishnu, to whom the month is dedicated.
The names in the solar calendar are just the names of the zodiac sign in which the sun travels. They are
  1. Mesha
  2. Vrishabha
  3. Mithuna
  4. Kataka
  5. Simha
  6. Kanyaa
  7. Tulaa
  8. Vrishcika
  9. Dhanus
  10. Makara
  11. Kumbha
  12. Miina

    Bahá'í calendar

The Bahá'í calendar is the calendar used by the Bahá'í Faith. It is a solar calendar with regular years of 365 days, and leap years of 366 days. Years are composed of 19 months of 19 days each, plus an extra period of "Intercalary Days". The months are named after the attributes of God. Days of the year begin and end at sundown.

Iranian calendar (Persian calendar)

The Iranian / Persian calendar, currently used in Iran and Afghanistan, also has 12 months. The Persian names are included in the parentheses. It begins on the northern Spring equinox.
  1. Farvardin
  2. Ordibehesht
  3. Khordad
  4. Tir
  5. Mordad
  6. Shahrivar
  7. Mehr
  8. Aban
  9. Azar
  10. Dey
  11. Bahman
  12. Esfand

    Reformed Bengali calendar

The Bangla calendar, used in Bangladesh, follows solar months and it has six seasons. The months and seasons in the calendar are:
No.Name Name Name SeasonDaysJulian months
1Boishakh BoishakhBoicákGrishmo 3114 April – May
2Joishtho ZoitZethGrishmo 31May – June
3Asharh AaŗAcárBorsha 31June – July
4Shrabon HaonCónBorsha 31July – August
5Bhadro BhadoBádoShorot 31August – September
6Aashin AshinAcínShorot 30September – October
7Kartik KhatiHatiHemonto30October – November
8Ogrohayon AghonÓonHemonto30November – December
9Poush PhushFucSheet 30December – January
10Magh Magh MakSheet 30January – February
11Falgun FagunFóonBoshonto 30 February – March
12Choitro SoitSoitBoshonto 30March – April

Nanakshahi calendar

The months in the Nanakshahi calendar are:
No.NamePunjabiDaysJulian months
1Chetਚੇਤ3114 March – 13 April
2Vaisakhਵੈਸਾਖ3114 April – 14 May
3Jethਜੇਠ3115 May – 14 June
4Harhਹਾੜ3115 June – 15 July
5Sawanਸਾਵਣ3116 July – 15 August
6Bhadonਭਾਦੋਂ3016 August – 14 September
7Assuਅੱਸੂ3015 September – 14 October
8Katakਕੱਤਕ3015 October – 13 November
9Magharਮੱਘਰ3014 November – 13 December
10Pohਪੋਹ3014 December – 12 January
11Maghਮਾਘ3013 January – 11 February
12Phagunਫੱਗਣ30/3112 February – 13 March

Khmer calendar

Like the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar. The solar is used more commonly than the lunar calendar. There are 12 months and the numbers of days follow the Julian and Gregorian calendar.
Julian and Gregorian nameKhmer nameTransliterationMeaningZodiac sign
JanuaryមករាMakaraមករ "naga"Capricorn
Februaryកម្ភៈKompeakក្អម "clay pitcher"Aquarius
Marchមិនា or មីនាMik Nea or Me Naត្រី "fish"Pisces
AprilមេសាMesaចៀម ពពៃ "ram"Aries
MayឧសភាUk Sak Pheaគោឈ្មោល "bull"Taurus
JuneមិថុនាMik Thok Naគូបុរសន"pair of boy & girl"Gemini
Julyកក្កដាKak Ka Daក្ដាម "crab"Cancer
AugustសីហាSeihaសីហៈ "lion"Leo
Septemberកញ្ញាKagnaស្រីក្រមុំ "girl"Virgo
OctoberតុលាTolaជញ្ជីង "scales"Libra
Novemberវិច្ឆិកាVichekaខ្ទួយ "scorpion"Scorpio
Decemberធ្នូThnuធ្នូ "bow, arc"Sagittarius

The Khmer lunar calendar contains 12 months; however, the eighth month is repeated every two or three years, making 13 months instead of 12.
English nameThai nameAbbr.TranscriptionSanskrit wordZodiac sign
Januaryมกราคมม.ค.mokarakhommakara "sea-monster"Capricorn
Februaryกุมภาพันธ์ก.พ.kumphaphankumbha "pitcher, water-pot"Aquarius
Marchมีนาคมมี.ค.minakhommīna " fish"Pisces
Aprilเมษายนเม.ย.mesayonmeṣa "ram"Aries
Mayพฤษภาคมพ.ค.phruetsaphakhomvṛṣabha "bull"Taurus
Juneมิถุนายนมิ.ย.mithunayonmithuna "a pair"Gemini
Julyกรกฎาคมก.ค.karakadakhomkarkaṭa "crab"Cancer
Augustสิงหาคมส.ค.singhakhomsiṃha "lion"Leo
Septemberกันยายนก.ย.kanyayonkanyā "girl"Virgo
Octoberตุลาคมต.ค.tulakhomtulā "balance"Libra
Novemberพฤศจิกายนพ.ย.phruetsachikayonvṛścika "scorpion"Scorpio
Decemberธันวาคมธ.ค.thanwakhomdhanu "bow, arc"Sagittarius

Tongan calendar

The Tongan calendar is based on the cycles of the moon around the earth in one year. The months are:
  1. Liha Mu'a
  2. Liha Mui
  3. Vai Mu'a
  4. Vai Mui
  5. Faka'afu Mo'ui
  6. Faka'afu Mate
  7. Hilinga Kelekele
  8. Hilinga Mea'a
  9. 'Ao'ao
  10. Fu'ufu'unekinanga
  11. 'Uluenga
  12. Tanumanga
  13. 'O'oamofanongo

    Kollam era ([Malayalam]) calendar

Malayalam nameTransliterationConcurrent Gregorian monthsSanskrit word and meaningZodiac sign
ചിങ്ങംchi-ngnga-mAugust–Septembersimha "lion"Leo
കന്നിka-nniSeptember–Octoberkanyā "girl"Virgo
തുലാംthu-lā-mOctober–Novembertulā "balance"Libra
വൃശ്ചികംvRSh-chi-ka-mNovember–Decembervṛścika "scorpion"Scorpio
ധനുdha-nuDecember–Januarydhanu "bow, arc"Sagittarius
മകരംma-ka-ra-mJanuary–Februarymokara "sea-monster"Capricorn
കുംഭംkum-bha-mFebruary–Marchkumbha "pitcher, water-pot"Aquarius
മീനംmee-na-mMarch–Aprilmīna " fish"Pisces
മേടംmE-Da-mApril–Maymeṣa "ram"Aries
ഇടവംi-Ta-va-mMay – Junevṛṣabha "bull"Taurus
മിഥുനംmi-thu-na-mJune–Julymithuna "a pair"Gemini
കർക്കടകംkar-kka-Ta-ka-mJuly–Augustkarkaṭa "crab"Cancer

Sinhalese calendar

The Sinhalese calendar is the Buddhist calendar in Sri Lanka with Sinhala names. Each full moon Poya day marks the start of a Buddhist lunar month. The first month is Vesak.
  1. Duruthu
  2. Navam
  3. Mædin
  4. Bak
  5. Vesak
  6. Poson
  7. Æsala
  8. Nikini
  9. Binara
  10. Vap
  11. Il
  12. Unduvap

    Germanic calendar

The old Icelandic calendar is not in official use anymore, but some Icelandic holidays and annual feasts are still calculated from it. It has 12 months, broken down into two groups of six often termed "winter months" and "summer months". The calendar is peculiar in that the months always start on the same weekday rather than on the same date. Hence Þorri always starts on a Friday sometime between January 22 and January 28 ', Góa always starts on a Sunday between February 21 and February 27 '.
  1. Gormánuður
  2. Ýlir
  3. Mörsugur
  4. Þorri
  5. Góa
  6. Einmánuður
  1. Harpa
  2. Skerpla
  3. Sólmánuður
  4. Heyannir
  5. Tvímánuður
  6. Haustmánuður

    Old Georgian calendar

MonthGeorgian Month NameTransliterationGeorgian Other NamesTransliteration
Januaryაპნისი, აპანიApnisi, Apani
JuneმარიალისაMarialisaთიბათვე, ივანობისთვეTibatve, Ivanobistve
JulyთიბისაTibisaმკათათვე, კვირიკობისთვეMkatatve, Kvirikobistve
NovemberტირისკონიTiriskoniგიორგობისთვე, ჭინკობისთვეGiorgobistve, Chinkobistve

*NOTE: New Year in ancient Georgia started from September.

Old Swedish calendar

  1. Torsmånad
  2. Göjemånad
  3. Vårmånad
  4. Gräsmånad
  5. Blomstermånad
  6. Sommarmånad
  7. Hömånad
  8. Skördemånad, Rötmånad
  9. Höstmånad
  10. Slaktmånad
  11. Vintermånad
  12. Julmånad

    Old English calendar

Like the Old Norse calendar, the Anglo-Saxons had their own calendar before they were Christianized which reflected native traditions and deities. These months were attested by Bede in his works On Chronology and The Reckoning of Time written in the 8th century. His months are probably those as written in the Northumbrian dialect of Old English which he was familiar with. The months were so named after the moon; the new moon marking the end of an old month and start of a new month; the full moon occurring in the middle of the month, after which the month was named.
  1. Æfterra-ġēola mōnaþ
  2. Sol-mōnaþ
  3. Hrēð-mōnaþ
  4. Ēostur-mōnaþ
  5. Ðrimilce-mōnaþ
  6. Ærra-Liþa
  7. Æftera-Liþa
  8. Weōd-mōnaþ
  9. Hāliġ-mōnaþ or Hærfest-mōnaþ
  10. Winter-fylleþ
  11. Blōt-mōnaþ
  12. Ærra-ġēola mōnaþ

    Old Hungarian calendar

Nagyszombati kalendárium from 1579.
Historically Hungary used a 12-month calendar that appears to have been zodiacal in nature but eventually came to correspond to the Gregorian months as shown below:
  1. Boldogasszony hava
  2. Böjtelő hava
  3. Böjtmás hava
  4. Szent György hava
  5. Pünkösd hava
  6. Szent Iván hava
  7. Szent Jakab hava
  8. Kisasszony hava
  9. Szent Mihály hava
  10. Mindszent hava
  11. Szent András hava
  12. Karácsony hava

    Czech calendar

  13. Leden – derives from 'led'
  14. Únor – derives from 'nořit'
  15. Březen – derives from 'bříza'
  16. Duben – derives from 'dub'
  17. Květen – derives from 'květ'
  18. Červen – derives from 'červená'
  19. Červenec – is the second 'červen'
  20. Srpen – derives from old Czech word 'sirpsti'
  21. Září – means 'to shine'
  22. Říjen – derives from 'jelení říje', which refers to the estrous cycle of female elk
  23. Listopad – falling leaves
  24. Prosinec – derives from old Czech 'prosiněti', which means to shine through

    Old Egyptian calendar

The ancient civil Egyptian calendar had a year that was 365 days long and was divided into 12 months of 30 days each, plus 5 extra days at the end of the year. The months were divided into 3 "weeks" of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events "wandered" through the calendar, it is referred to as Annus Vagus or "Wandering Year".
  1. Thout
  2. Paopi
  3. Hathor
  4. Koiak
  5. Tooba
  6. Emshir
  7. Paremhat
  8. Paremoude
  9. Pashons
  10. Paoni
  11. Epip
  12. Mesori

    Nisga'a calendar

The Nisga'a calendar coincides with the Gregorian calendar with each month referring to the type of harvesting that is done during the month.
  1. K'aliiyee = Going North – referring to the Sun returning to its usual place in the sky
  2. Buxwlaks = Needles Blowing About – February is usually a very windy month in the Nass River Valley
  3. Xsaak = To Eat Oolichans – Oolichans are harvested during this month
  4. Mmaal = Canoes – The river has defrosted, hence canoes are used once more
  5. Yansa'alt = Leaves are Blooming – Warm weather has arrived and leaves on the trees begin to bloom
  6. Miso'o = Sockeye – majority of Sockeye Salmon runs begin this month
  7. Maa'y = Berries – berry picking season
  8. Wii Hoon = Great Salmon – referring to the abundance of Salmon that are now running
  9. Genuugwwikw = Trail of the Marmot – Marmots, Ermines and animals as such are hunted
  10. Xlaaxw = To Eat Trout – trout are mostly eaten this time of year
  11. Gwilatkw = To Blanket – The earth is "blanketed" with snow
  12. Luut'aa = Sit In – the Sun "sits" in one spot for a period of time

    French Republican calendar

This calendar was proposed during the French Revolution, and used by the French government for about twelve years from late 1793. There were twelve months of 30 days each, grouped into three ten-day weeks called décades. The five or six extra days needed to approximate the tropical year were placed after the months at the end of each year. A period of four years ending on a leap day was to be called a Franciade. It began at the autumn equinox:
  1. Vendémiaire
  2. Brumaire
  3. Frimaire
  1. Nivôse
  2. Pluviôse
  3. Ventôse
  1. Germinal
  2. Floréal
  3. Prairial
  1. Messidor
  2. Thermidor
  3. Fructidor

    Eastern Ojibwe calendar

month names are based on the key feature of the month. Consequently, months between various regions have different names based on the key feature of each month in their particular region. In the Eastern Ojibwe, this can be seen in when the sucker makes its run, which allows the Ojibwe to fish for them. Additionally, in the Eastern Ojibwa-Chippewa-Ottawa Dictionary, Dr. Richard Rhodes also informs of not only the variability in the month names but how in the Eastern Ojibwe, these names were originally applied to the lunar months the Ojibwe originally used, which was a lunisolar calendar marked by the moon, fixed to Akiinaaniwan that marks when sunrise is the latest in the Northern Hemisphere. Due to Eastern Ojibwe is a vowel syncope dialect, the elided vowels have been added back in the table below, shown in brackets.
MonthMonth in Eastern OjibweTranslationOriginally the month of the Ojibwa yearStarting at the first full moon after:
in those places that do not have a sucker run during that time
shki-bboon-giisoonsLittle New-winter moon13 used if there is a new moon after gchi-bboon-giizis before December 27.
in those places that have a sucker run during that time
nmebin-giizisSucker moon1December 27
in those places that have a sucker run during that time
nmebni-giizisSucker moon1December 27
Februarynaabni-giizisCrust-on-the-snow moon2January 25
Marchziizbaakdoke-giizisSugaring moon3February 26
in those places that have a sucker run during that time
nmebin-giizisSucker moon4March 25
in those places that have a sucker run during that time
nmebni-giizisSucker moon4March 25
in those places that do not have a sucker run during that time
waawaasgone-giizisFlower moon4March 25
in those places that have an April sucker run
waawaasgone-giizisFlower moon4March 25
in those places that have a January sucker run
gtige-giizisPlanting moon5April 24
in those places that have an April sucker run
gtige-giizisPlanting moon5April 24
in those places that have a January sucker run
dehmin-giizisStrawberry moon6May 23
Julymiin-giizisBlueberry moon7June 22
Augustdatgaagmin-giizisBlackberry moon8July 20
Septembermdaamin-giizisCorn moon9August 18
Octoberbnaakwe-giizisLeaves-fall moon10September 17
Octoberbnaakwii-giizisHarvest moon10September 17
Novembergshkadin-giizisFreeze-up moon11October 16
Decembergchi-bboon-giizisBig-winter moon12November 15