Because the Islamic calendar is based on the lunar month, which is roughly 29 days, it usually ends up being about 11 days shorter than the Western calendar. The calendar used in the West, the Gregorian or Christian calendar, is a solar calendar that is based on the earth's 365-day year.
Because the Western calendar is based on an Earth year, its months are divided somewhat arbitrarily, and February is a month with an irregular number of days. By contrast, the Islamic year is 354 or 355 days long based on the moon's cycle, and each month is either 29 or 30 days long.
The Islamic calendar consists of 12 months: Muharram, Safar, Rabi' al-Awwal, Rabi' al-Thani, Jumada al-Ooola, Jumada al-Ukhra, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Qa'dah and Dhu al-Hijjah. The months of Jumada al-Oola and Jumada al-Ukhra may also be known as Jumada al-Awwal and Jumada al-Thani, respectively.
The Islamic calendar was introduced during the Western year 632 C.E. It is officially used in some parts of the world, and in places where the Western calendar is official, Muslims still make use of the Islamic calendar in order to determine the dates of important holidays and religious events.