The Islamic Hijri calendar has a 11 leap years in a 30-year cycle. An extra day is added to the last month of the year during the Islamic leap year.

A leap day is added

During a leap year 1 day is added to the last month, making it 30 days instead of 29 days, in a leap year. This month, Dhu 'l-Hidjdja, is also referred to as the month of the Hajj – the Muslim pilgrimage to Mecca. The Hijri calendar has a 30-year cycle with 11 leap years of 355 days and 19 years of 354 days. In the long term, it is accurate to about one day in 2500 years.

The leap year occurs in the 2nd, 5th, 7th, 10th, 13th, 16th, 18th, 21st, 24th, 26th and 29th years of the 30-year cycle. Leap months are forbidden by the Qur'an. The calendar is based on the Qur'an and its proper observance is a sacred duty for Muslims. It is a purely lunar calendar and contains 12 months that are based on the moon’s motion.