The Islamic calendar is based on the lunar cycle, which means that it follows the phases of the moon. This differs from the Gregorian calendar, which is based on the solar cycle. The Islamic calendar consists of twelve lunar months, each of which is approximately 29.5 days long. As a result, the Islamic year is roughly 11 days shorter than the Gregorian year. One important feature of the Islamic calendar is that it determines the dates of important religious events, such as Ramadan and Eid al-Fitr, which are celebrated by Muslims around the world.