In Today's meaning:

The advent calendar is a way to count down the days to Christmas! This is used to let your kids or family members/friends do fun things until Christmas day. This is not really related to Christ anymore.

Advent Calendars are very different and they all go up to Christmas Eve! You get to open one every day, and this is very fun to most people.

History:

Advent calendars can be traced back to the 19th century when families would mark every day in December until Christmas Eve with a chalk line. Originally, the four-week period known as Advent began as a time for converts to Christianity to prepare for baptism.