This day commemorates Ibrahim’s (Abraham) willingness to sacrifice his son to God. This festival also marks the end of the Hajj pilgrimage to Mecca.

The issue with this holiday is that it is on a different day ever year. (lately it has been between June and September) Regional customs or moon sightings may cause a variation of the date for Islamic holidays, which begin at sundown the day before the date specified for the holiday. The Islamic calendar is lunar and the days begin at sunset, so there may be one-day error depending on when the New Moon is first seen.

It sounds complicated, but I guess many Christian and Jewish celebrations are complicated too.