The Islamic calendar is the religious calendar used by Muslims to determine the proper days to observe religious events such as Ramadan, attend hajj and celebrate other holidays.

It is used in most Muslim countries concurrently with the Gregorian calendar.

It consists of 12 months, each month alternatively having 29 or 30 days. In total it has 354 or 355 depending when the crescent moon is first observed. This means that the year does not correspond to the solar year used by the Gregorian system. It is also difficult to predict what the future calendar will be, because the year does not begin with the actual position of the moon, but when it is first observed.

Each month has a specific name based on a variety of Islamic and pre-Islamic Arabic roots. Four months are consider sacred and fighting is prohibited during these months, though there is disagreement of which specific months. This was traditionally due to allow for peaceful passage to Mecca for the pilgrimage.

The Islamic calendar years begin from 622 AD when the Prophet Muhammed emigrated from Mecca to Medina, an event known as the Hijra. This is why it is alternatively called the Hijri Calendar.