At sunset today (as I write, Wednesday, January 23, 2013), Muslims around the world will begin their observance of the birth of the Prophet Muhammad, an observance known in the Islamic world as Mawlid al-Nabi (Arabic for “Birth of the Prophet”), or simply Mawlid for short.

Mawlid falls each year upon the 12th day of the month known as Rabi al-Awwal, which is the third month of the year in the Islamic calendar. Unlike the Western (Gregorian) calendar, which is a solar calendar, the Islamic calendar is a lunar calendar; it calculates its months in a different manner, and its year is 10 to 11 days shorter than the 365-day year of the Gregorian calendar. This means that a certain amount of “drift” accumulates each year between the two calendars.

Consequently, while Islamic holidays (such as Eid al-Adha and Eid al-Fitr, in addition to Mawlid) and other calendar dates always fall within the same months and upon the same dates on the Islamic calendar each year, they do not always fall within the same months or upon the same dates on the Western or Gregorian calendar. (The same is true of the Hebrew or Jewish calendar, which is also a lunar calendar, albeit one of a different sort than the Islamic calendar; so, Jewish holidays such as Rosh Hashanah or Yom Kippur or Hanukkah likewise do not fall on the same dates every year on the Gregorian calendar, but vary somewhat.)

Therefore, by way of example, last year Mawlid began at sunset on February 3, 2012; next year, by contrast, Mawlid will begin at sunset on January 13, 2014. This year, however, Mawlid begins on January 23, 2013 — and at sunset.

Why at sunset? Because Islam traditionally reckons the start of each new day as beginning not at midnight, or at dawn, but instead at sunset or nightfall. (This is another practice that both Islam and Judaism share in common; and this is why, for instance, the Jewish sabbath or shabbat technically begins at sunset each Friday.)

Of course, like every major religion, Islam is not monolithic or homogeneous; Islam is internally diverse, containing internal divisions and sectarian subdivisions within itself. The primary such division within Islam is between the Sunni majority (about 85% to 90% of Muslims worldwide) and the Shia minority (about 10% to 15%). Sunni Muslims observe Mawlid on the 12th day of the third Islamic calendar month; however, Shiite Muslims instead observe Mawlid on the 17th day of that month. So, whereas for Sunnis Mawlid begins at sunset tonight (on January 23), for Shiites Mawlid will instead begin at sunset on January 28.

Mawlid is officially celebrated as a public holiday in most Islamic countries, but not in all. One particularly notable exception is Saudi Arabia, whose conservative national religious leadership disapproves of the holiday’s celebration as an impious, un-Quranic, and impermissible innovation or break with tradition. Elsewhere throughout the Islamic world, some additional conservative sectarian religious leaders have also issued calls not to celebrate Muhammad’s birthday, though their voices appear to be a largely ignored minority in many such places. For many if not most Muslims, it seems that Mawlid is uncontroversial, a joyous and fully permissible celebration of the birthday of their holy Prophet.