The Church Year consists of two cycles of feasts and holy days: one is dependent upon the movable date of the Sunday of the Resurrection or Easter Day; the other, upon the fixed date of December 25, the Feast of our Lord's Nativity or Christmas Day.
Easter Day is always the first Sunday after the full moon that falls on or after March 21. It cannot occur before March 22 or after April 25.
The sequence of all Sundays of the Church Year depends upon the date of Easter Day. The date of Easter also determines the beginning of Lent on Ash Wednesday, and the feast of the Ascension on a Thursday forty days after Easter Day.
But the Sundays of Advent are always the four Sundays before Christmas Day, whether it occurs on a Sunday or a weekday.