There are two official holidays in Islam: Eid Al-Fitr and Eid Al-Adha. Eid Al-Fitr is celebrated at the end of Ramadan (a month of fasting during daylight hours), and Muslims usually give zakat (charity) on the occasion. Eid Al-Adha is celebrated on the tenth day of Dhu al-Hijjah and lasts for three days, during which Muslims usually sacrifice a sheep and distribute its meat in 3 parts: among family, friends, and the poor.
Both of the holidays occur on dates in the Islamic (Hijri) calendar, which is lunar, and thus their dates in the Gregorian calendar, which is solar, change each year. The Gregorian calendar is based on the orbital period of the Earth’s revolution around the Sun, approximately 365 1⁄4 days, while the Islamic calendar is based on the synodic period of the Moon’s revolution around the Earth, approximately 29 1⁄2 days. The Islamic calendar alternates months of 29 and 30 days (which begin with the new moon). Twelve of these months constitute an Islamic year, which is 11 days shorter than the Gregorian year.