The church year or church calendar (used interchangeably) has two parts, like a large circle divided in two.
One arc of the circle places the beginning of the church year on the first Sunday of Advent four weeks before Christmas. This period stretches until Pentecost in May or June and tells the story of the life of Christ.
The second arc of the circle goes from Pentecost until Christ the King Sunday at the end of November, and tells the story of the life of the church.
As Christians anchored to the Kingdom of God, it makes sense we should live our days in this way and find the benefits afforded from slowing down to not simply count the days, but make the days count.