Counting the days of Advent can often be confusing, as most modern "Advent Calendars," start on December 1 and end on December 24. While on occasion this might line-up with the Catholic liturgical season of Advent, typically it is either longer or shorter than a simple December calendar.
In the Roman Rite of the Catholic Church, the length of Advent is fixed to the four Sundays before Christmas.
This type of calculation means that the number of days varies year-to-year.
Advent could begin as early as November 27, giving the liturgical season a total of 28 days. It could also begin as late as December 3, shortening the season of preparation to only 21 days.
Regardless of when it begins, Advent in the Roman Rite is always shorter than Lent, which has a total of 40 days. This isn't the case in other Eastern Christian traditions, who observe a full 40 day fast prior to Christmas.
