The Islamic calendar is called Hijrah (Migration) calendar.
It begins from the day when Muhammad (Peace be upon him) made a Hijrah from Makkah to Madinah.
The Islamic calendar was implemented many years later, retroactively, from July 16, 622 CE, by 'Umar bin Khattab (May God be pleased with him).
Years of the Islamic calendar are approximately ten days shorter than the Gregorian calendar of 365 days, we normally use. Each month of the Islamic calendar could be either 29 or 30 days long, depending upon the moon cycle. First day of the month starts after the sighting and visibility of the moon.
The beginning of the month of fasting Ramadan (Ninth month of the Islamic calendar), ’Id al-Fitr (Festival of Fasting) at the end of Ramadan and Hajj (Pilgrimage to Ka’bah) are always announced only after sighting the moon.
Compulsory Charity (Zakah) is also paid according to the Islamic calendar.
The twelve months of the Islamic Calendar are:
3. Rabi’ ul Awwal
4. Rabi’ uth-thani
5. Jumadal ula
6. Jumadal Ukhra
11. Dhul Qa'dah
12. Dhul Hijjah