Common year starting on Friday


A common year starting on Friday is any non-leap year that begins on Friday, 1 January, and ends on Friday, 31 December. Its dominical letter hence is C. The most recent year of such kind was 2021, and the next one will be 2027 in the Gregorian calendar, or, likewise, 2022 and 2033 in the obsolete Julian calendar; see [|below for more]. This common year is one of the three possible common years in which a century year can end on, and occurs in century years that yield a remainder of 100 when divided by 400. The most recent such year was 1700, and the next one will be 2100.
Any common year that starts on Friday has only one Friday the 13th: the only one in this common year occurs in August. Leap years starting on Thursday share this characteristic, but also have another one in February.
From July of the year that precedes this type of year until September in this type of year is the longest period. This type of year also has the longest period without a Tuesday the 13th, from July of this year until September of the next common year, unless the next year is a leap year, then the period is reduced to only 11 months.
From February until March in this type of year is also the shortest period that runs between two months that begin exactly on the first day of the week, in areas where Monday is regarded the first day of the week.
This is the one of two types of years overall where a rectangular February is possible, in places where Monday is considered to be the first day of the week. Common years starting on Thursday share this characteristic, when Sunday is considered to be the first day of the week.
Additionally, this type of year has three months beginning exactly on the first day of the week, in areas which Monday is considered the first day of the week. Leap years starting on Monday share this characteristic on the months of January, April and July.
This year has two months which begin on a weekend-day. Since at least one month begins on each day of the week in all years, this is the fewest possible number of months to begin on a weekend-day in a given year; this also occurs in a leap year starting on Friday, in which case the two months are May and October.

Calendars

This is the only year type where the nth "Doomsday" is not in ISO week n; it is in ISO week n-1.

Applicable years

Gregorian calendar

In the Gregorian calendar, alongside Sunday, Monday, Wednesday or Saturday, the fourteen types of year repeat in a 400-year cycle. Forty-three common years per cycle or exactly 10.75% start on a Friday. The 28-year sub-cycle only spans across century years divisible by 400, e.g. 1600, 2000, and 2400.
For this kind of year, the ISO week 10 and all subsequent ISO weeks occur later than in all other years, and exactly one week later than Leap years starting on Thursday. Also, the ISO weeks in January and February occur later than all other common years, but leap years starting on Friday share this characteristic in January and February, until ISO week 8.
0–9910212738495566778394
100–199100106117123134145151162173179190
200–299202213219230241247258269275286297
300–399309315326337343354365371382393399

Julian calendar

In the now-obsolete Julian calendar, the fourteen types of year repeat in a 28-year cycle. This sequence occurs exactly once within a cycle, and every common letter thrice.
As the Julian calendar repeats after 28 years that means it will also repeat after 700 years, i.e. 25 cycles. The year's position in the cycle is given by the formula + 1). Years 4, 15 and 26 of the cycle are common years beginning on Friday. 2017 is year 10 of the cycle. Approximately 10.71% of all years are common years beginning on Friday.

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