Islam follows the lunar calendar, so the start of Eid is determined by the sighting of the moon. Once the crescent moon marking the start of the next month, Shawwal, is spotted, the month of Ramadan is over and Eid may begin. This means that, according to the solar Gregorian calendar used by the secular world, Eid falls on a different day every year.
Another consequence of the lunar start date is that the beginning of Eid may vary from country to country, depending on when the moon is sighted. While most Muslim countries began celebrating Eid today, Oman did not sight the crescent moon and will begin celebrating tomorrow instead, Al Arabiya reports.