The Islamic calendar (or Hijri calendar) is based on lunar months- a cyclical nature that relates to a natural phenomenon – like the Jewish calendar. It contains 12 months of 29 or 30 days each, for a total of 354 days long or 355 days long.
Because it is based on the motion of the moon, when a thin new crescent Moon is sighted in the western sky after sunset within a day or so after the New Moon, thus the first day of the month is usually the second day after the astronomical New Moon.
The Islamic calendar is consistently shorter (11 Days) than a solar year, and therefore it shifts with respect to the Gregorian calendar. Unlike its Jewish counterpart, however, the Islamic calendar has no corrective system to align it with the solar calendar. Thus the Islamic holidays do not always fall in the same season, and they occur earlier by 11 days every year on the solar calendar.
The months of the Islamic calendar are as follows:
3. Rabi’ al-awwal (Rabi’ I)
4. Rabi’ al-thani (Rabi’ II)
5. Jumada al-awwal (Jumada I)
6. Jumada al-thani (Jumada II)
11. Dhu al-Qi’dah
12. Dhu al-Hijjah
The Islamic calendar is the official calendar in countries around the Persian Gulf, especially The Kingdom of Saudi Arabia. But other Muslim countries use both the Gregorian calendar for civil purposes because all dates are based on the Gregorian calendar and turn to the Islamic calendar for religious purposes.