Isru chag

Isru Chag refers to the day after each of the Three Pilgrimage Festivals in Judaism: Passover, Shavuot and Sukkot.
The phrase originates from the verse in, which states, "Bind the festival offering with cords to the corners of the altar." According to the Talmud:
In a responsum to a community that had inquired as to the rationale behind the observance of Isru Chag, Yosef Hayyim cited Isaac Luria, the founder of Lurianic Kabbalah, to the effect that Jews connect the day after the holiday to the holiday itself due to the remaining “light” of the holiday: in other words, so that the sanctity of the holiday will be extended.

Rabbinic authorities cite observances

Adding a degree of festivity to the day as a practice has further been codified in Ashkenazic communities, as Moses Isserles has stated in his glosses on the Shulchan Aruch in the section Orach Chayim:
Yisrael Meir Kagan ruled that the minhag is to generally forbid fasting on Isru Chag, except in instances when as a result of great distress the community synagogue decrees it.
Almost all communities omit tachanun on Isru Chag. However, communities that follow the rulings of Maimonides, such as the Dor Daim, maintain that the only days on which Tachanun is to be omitted are Shabbat, Jewish holidays, Rosh HaShanah, Rosh Chodesh, Hanukkah, Purim, and the mincha on the eve of any Shabbat and holiday.