The Islamic calendar, Muslim calendar or Hijri calendar (AH) is a lunar calendar consisting of 12 months in a year of 354 or 355 days. It is used to date events in many Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper days on which to observe the annual fast (see Ramadan), to attend Hajj, and to celebrate other Islamic holidays and festivals. The first year was the Islamic year beginning in AD 622 during which the emigration of the Islamic Prophet Muhammad from Mecca to Medina, known as the Hijra, occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra); hence, Muslims typically call their calendar the Hijri calendar. The Islamic calendar is now used primarily for religious purposes, and for official dating of public events and documents in Muslim countries. The Islamic calendar is not to be confused with a lunar calendar that is based on astronomical calculations. The latter is based on a year of 12 months adding up to 354.37 days. Each lunar month begins at the time of the monthly "conjunction", when the Moon is located on a straight line between the Earth and the Sun. The month is defined as the average duration of a rotation of the Moon around the Earth (29.53 days). By convention, months of 30 days and 29 days succeed each other, adding up over two successive months to 59 full days. This leaves only a small monthly variation of 44 minutes to account for, which adds up to a total of 24 hours (i.e. the equivalent of one full day) in 2.73 years. To settle accounts, it is sufficient to add one day every three years to the lunar calendar, in the same way that one adds one day to the Gregorian calendar, every four years.

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Eid al-Adha, also called Feast of the Sacrifice, the Major Festival, the Greater Eid, Kurban Bayram, is the second of two religious holidays celebrated by Muslims worldwide each year. It honours the willingness of Abraham (Ibrahim) to sacrifice his young first-born son Ishmael (Ismail])a as an act of submission to God's command and his son's acceptance to being sacrificed, before God intervened to provide Abraham with a lamb to sacrifice instead. In the lunar based Islamic calendar, Eid al-Adha falls on the 10th day of Dhu al-Hijjah and lasts for four days. In the international Gregorian calendar, the dates vary from year to year, drifting approximately 11 days earlier each year. Eid al-Adha is the latter of the two Eid holidays, the former being Eid al-Fitr. The basis for the Eid al-Adha comes from the 196th ayah (verse) of Al-Baqara, the second sura of the Quran. The word "Eid" appears once in Al-Ma'ida, the fifth sura of the Quran, with the meaning "solemn festival". Like Eid al-Fitr, Eid al-Adha begins with a Sunnah prayer of two rakats followed by a sermon (khutbah). Eid al-Adha celebrations start after the descent of the Hajji from Mount Arafat, a hill east of Mecca. Ritual observance of the holiday lasts until sunset of the 12th day of Dhu al-Hijjah. Eid sacrifice may take place until sunset on the 13th day of Dhu al-Hijjah.The days of Eid have been singled out in the Hadith as "days of remembrance". The days of Tashriq are from the Fajr prayer of the 9th of Dhul Hijjah up to the Asr prayer of the 13th of Dhul Hijjah (5 days and 4 nights). This equals 23 prayers: 5 on the 9th-12th, which equals 20, and 3 on the 13th. Source

Source: Wikipedija

Ramadan is the ninth month of the Islamic calendar, and the month in which the Quran was revealed. Fasting during the month of Ramadan is one of the Five Pillars of Islam. The month is spent by Muslims fasting during the daylight hours from dawn to sunset.The Quran was sent down to the lowest heaven during this month, thus being prepared for gradual revelation by Jibreel (Gabriel) to the Islamic prophet Muhammad. Furthermore, Muhammad told his followers that the gates of Heaven would be open for the entire month and the gates of Hell (Jahannam) would be closed. The first day of the next month, Shawwal, is spent in celebration and is observed as the "Festival of Breaking Fast" or Eid al-Fitr. The Islamic calendar is a lunar calendar, and months begin when the first crescent of a new moon is sighted. Since the Islamic lunar calendar year is 10 to 11 days shorter than the solar year and contains no intercalation, Ramadan migrates throughout the seasons. The Islamic day starts after sunset. 

Source:Wikipedia