Islamic lunar calendar

The Islamic, Muslim, or Hijri calendar (Arabic: التقويم الهجري‎‎ at-taqwīm al-hijrī) is a lunar calendar consisting of 12 months in a year of 354 or 355 days. It is used (often alongside the Gregorian calendar) to date events in many Muslim countries. It is also used by Muslims to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca.

The Islamic calendar employs the Hijri era whose epoch was retrospectively established as the Islamic New Year of AD 622. During that year, Muhammad and his followers migrated from Mecca to Yathrib (now Medina) and established the first Muslim community (ummah), an event commemorated as the Hijra. In the West, dates in this era are usually denoted AH (Latin: Anno Hegirae, "in the year of the Hijra") in parallel with the Christian (AD) and Jewish eras (AM). In Muslim countries, it is also sometimes denoted as H from its Arabic form ( سَنة هِجْريّة, abbreviated ). In English, years prior to the Hijra are reckoned as BH ("Before the Hijra").

...
Wikipedia