The name is reminiscent of an advent calendar, which traditionally has a labeled door behind which is a piece of candy, small toy or other trinket. These calendars are intended for young children to remind them of the period of advent.

If you are religious, look upon this as a similar thing which reminds you of the season, building in anticipation of the final day.

If you are not religious, it's just a word with no special meaning other than the time of year. Same goes if you are not of a Christian religion.