The Muslim calendar is lunar-based with 12 months of 29 or 30 days and a year of 354 days. A lunar year is shorter than a solar year, so Muslim holydays cycle backwards through the Western calendar. The Islamic lunar calendar differs from the moon is actual cycle by one day every 2,570 years, so it is only a little less accurate than the solar calendar.
Sunnis and Shi'a each have they're own dates for observing Mawlid: this is the Shi'a date.
mawlid an-nabi, sometimes simply called in colloquial Arabic, mawlid, is a term used to refer to the observance of the birthday of the Islamic prophet Muhammad which occurs in Rabi' al-awwal, the third month in the Islamic calendar Muslim scholars differ on whether observing the Prophet's birthday is appropriate or not.