The Festival of Nine Lessons and Carols, as it is formally called, is traditionally used at Christmas. The story of the fall of humanity, the promise of the Messiah, and the birth of Jesus is told in nine Bible readings selected from Book of Genesis, the prophetic books and the four Gospels, interspersed with the singing of Christmas carols, hymns and choir music.
The format was based on an Order drawn up by the Bishop of Truro in Cornwall, Edward White Benson, who later became the Archbishop of Canterbury. Lessons and Carols were first celebrated on Christmas Eve 1880. Tradition says that the bishop organized the service in a small wooden chapel on Christmas Eve with the goal of giving men something to do other than go to the pubs before the midnight service.
The service has since become a tradition as a pre-Christmas liturgy with the most famous version being broadcast from King’s College in Cambridge on Christmas Eve. https://anglican.ca/amo/news/lessons-and-carols/