Why did the length of advent change?

The length of Advent has changed primarily due to shifts in liturgical practice and the calendars used by different Christian denominations. Traditionally, Advent lasts four weeks, beginning four Sundays before Christmas. However, some traditions may adjust its length based on the specific calendar year or liturgical focus, such as emphasizing certain feast days. Additionally, variations in the interpretation of the Advent season and its significance have led to different observances across various Christian communities.