As Advent approaches, this article from The Lutheran Witness serves as a great explanation of why we observe the liturgical year:
We celebrate the Church Year in part as a celebration of time. Time is God’s gift, and God’s creation sets the rhythms that make time possible. The Church Year is a way of marking time and making time. It is a way of telling time and keeping time. And what is it that we mark and make and tell and keep?
We mark the events of God acting in world history and in our own personal history. We make a place to gather together, to remember and give thanks for what God has done for us. We tell the story of how God is working in our midst. And we keep the vigil for what God will yet do among His people.
The Church Year is for all these things. It is for telling the story of how a God beyond time acts within time to save and restore life. It uses the months and seasons of the calendar to tell the story. Or rather, it tells the story in the seasons of the year and our own lives.