Advent is about preparing for the coming of Jesus, both in remembrance of his birth and in anticipation of his return.
The Festival of Nine Lessons and Carols has been offered annually by the choir of King's College in Cambridge, England since 1918, and similar "Lessons and Carols" services have been offered by churches worldwide since the mid-20th century. While this service is relatively recent, it's basic purpose and structure dates back to at least the fourth century.