The Prophets Bithday
Note that in the Muslim calander, a holiday begins on the sunset of the previous day, so observing Muslims will celebrate Mawlid al-Nabi on the sunset.
Although Mawlid al-Nabi is always on the same day of the Islamic calendar, the date on the Gregorian calendar varies from year to year, since the Gregorian calendar is a solar calendar and the Islamic calendar is a lunar calendar. This difference means Mawlid al-Nabi moves in the Gregorian calendar approximately 11 days every year. The date of Mawlid al-Nabi may also vary from country to country depending on whether the moon has been sighted or not.