You may have noticed that for each season of the church, there are slightly different titles for the Sundays in them:
- The Sundays of Advent
- The Sundays after Christmas
- The Sundays after the Epiphany
- The Sundays in Lent
- The Sundays of Easter
- The Sundays after Pentecost
"After" is reserved for those Sundays in Seasons that fall after major days. This includes the Christmas Season, the Season after the Epiphany, and the Season after Pentecost. What these Seasons all have in common is they vary in how many Sundays are in them.
Other than that, these seasons are very different from one another. We see this first in the days they are named for. Christmas and Epiphany are on set days of the year, December 25 and January 6 respectively; Pentecost is not. Additionally, the Seasons after the Epiphany and Pentecost vary each year in length, while the Christmas Season is always 12 days. The Seasons of the Epiphany and Pentecost also mark the times between Advent/Christmas and Lent/Easter.
"In" marks how Sundays are viewed in the Season of Lent. Lent is 40 days of fasting to prepare for Easter, though if we counted Sundays, it would actually be 46 days from Ash Wednesday until Easter. The "in" is a sign that these Sundays are a break from our fast and that every Sunday is always a feast of the Resurrection really.
"Of" means that the Sundays of these seasons are fully meant to be a part of those seasons. While Advent is a time of preparation like Lent, there is not the same call to fasting that we see in the Ash Wednesday liturgy at the start of Lent. The "of" in Easter reminds us that Easter is not a day, but an entire season.
Each season works a little differently. The differences in the titles of the Sundays in each of those seasons shows that.