According to Wiki, the Prophet Muhammad (sallallahu `alayhi wa sallam) arrived in Medina on this day (on the Western calendar) in 622 CE, completing the Hijra, or “withdrawal,” from Mecca, the city in which he lived when first receiving the revelations of the Qur’an, to Medina (then called Yathrib), the city in which he sought shelter from persecution at the hands of the established powers in Mecca. This event is used to mark Year 1 in the Islamic calendar as it marks the beginning of a unified Muslim political community.

Of course the Hijra is usually celebrated by the Islamic calendar– on the 8th day of Rabi’ al-awwal, the third month of the year. Since the Islamic calendar is about 11 days shorter than the Western calendar, dates on the Islamic calendar will move throughout the seasons and through the Western calendar.

This difference leads to much hilarity, as the dates of important Islamic events (the start of Ramadan, Eid-al-fitr, etc.) change from (Western) year to year– so, for example, your college might just cut and paste old Ramadan dates onto its new calendar, thus getting the dates wrong, or it might schedule an Islamic Studies lecture on Eid. Hypothetically.

(The other hilarity-generating feature of the Hijra calendar is that it’s lunar, and some old-school types insist on having moon sightings determine the start and end of months, while other people think we should just calculate. So there’s always disagreement about just when Ramadan starts, when the holidays are, and so on. As if Christmas might be the 25th, might be the 26th, won’t know until it’s here. The Ummah Films guy has a funny riff about this in his Ramadan video at about 1:00– these arguments really do get kind of heated.)