The Muslim or Hijri calendar comprises 12 months and 354 days, and it follows the lunar cycle. Each month bears 29 or 30 days depending on the visibility of the moon, the position of the earth and weather conditions. The first day of each month officially begins on the birth of a new lunar cycle determined by the appearance of the crescent moon.
The Muslim calendar utilises the initials A.H., which stands for the Latin phrase Anno Hegirae and is the equivalent of A.D. in the Gregorian calendar. Muslims count the years beginning with A.D. 622, when Muhammad escaped from Mecca to Medina. Because the lunar calendar is behind the Gregorian calendar by approximately 10 days each year, months of the Muslim calendar fall in different parts of the Gregorian calendar in a cycle that repeats after 33 years.
Muslim countries use the Hijri calendar to date events and determine the proper days on which to observe Ramadan, attend Hajj and celebrate other Islamic holidays. Four of the 12 months of the calendar are sacred. These are Rajab, Dhu al-Qa'dah, Dhu al-Hijjah and Muharram.
Umar ibn Al-Khattab, a follower of Muhammad, introduced the Muslim calendar in 638 A.D., 17 years after the Hegira, which is the time when Muhammad emigrated to Medina. The Muslim calendar is the official calendar in most predominantly Muslim countries.