The Muslim (Islamic) calendar is the lunar calendar used in Islam to determine the dates of religious holidays, and also as the official calendar in Saudi Arabia, the Persian Gulf countries, and some other Muslim countries. The calendar is based on Hijra (July 16, 622) - the date of the migration of the Prophet Muhammad and the first Muslims from Mecca to Medina. Therefore, in Muslim countries, the calendar is called the Hijra calendar (Arabic: الكلمان الهجري). The era before the beginning of the calendar is called Jahiliyyah - the era of ignorance.
The Muslim calendar is built exclusively on the changing phases of the moon. According to tradition, the first day of the new month was the first day after the new moon, when a crescent moon could be seen in the sky shortly after sunset. Moreover, two respectable Muslims had to witness the appearance of the moon. Because of this, the beginning of the new month, as well as religious holidays, could not coincide in different Islamic countries, because due to the difference in time between sunset at different geographical latitudes, the conditions of visibility of the new moon may also be different.
Since the lunar synodic period is approximately 29.5 days, the months of the Islamic calendar are either 29 or 30 days long. Additional days of the month are prohibited, except for the single day that serves to contain the new moon near the 1st of the month. This day is included in the last month of Dhul-Hijjah, which can have 29 or 30 days. In the latter case, the year is called a leap year.
Currently, the so-called "Arab cycle" and "Turkish cycle" are used to calculate leap years. The Arabic cycle is based on a 30-year sequence and every 2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29th year of the cycle are considered leap years. In the eight-year "Turkish cycle", the 2nd, 5th and 7th years of the cycle are considered leap years. The leap years of the 8-year cycle do not coincide with the leap years of the 30-year cycle.
One year consists of 12 months and has 354 or 355 days. Unlike a number of other lunar calendars (such as the Babylonian), an additional thirteenth month is not introduced to align the lunar calendar with the solar cycle, as it is believed to be prohibited by the Koran. In this way, the beginning of the year and the days of Muslim religious holidays are gradually shifted in time