"Indeed, the number of months with Allah is twelve months in the register of Allah from the day He created the heavens and the earth; of these, four are sacred."
— Quran 9:36
About the Hijri Calendar
The Islamic (Hijri) calendar is a lunar calendar of 12 months based on the cycles of the moon. Each month begins with the sighting of the new crescent moon. A Hijri year is approximately 354 days, making it 10-11 days shorter than the Gregorian year. The calendar begins from the year of the Prophet's ﷺ migration (Hijrah) from Makkah to Madinah in 622 CE.
Months per year
12
Days per year
~354
The Four Sacred Months
Muharram, Rajab, Dhul Qa'dah, and Dhul Hijjah are the four sacred months in which warfare was traditionally prohibited and good deeds carry extra weight.