We are waiting. We are preparing.
At each Mass during Advent, the theme is one of getting ready for the feast. It is not yet time to shout and celebrate. That time will come, and we’ll be ready for it. But singing Christmas Carols at Mass during Advent would be similar to singing “Jesus Christ Is Risen Today, Alleluia!” during Lent. Doesn't make sense.
The season of Advent is about how Isreal waited for Jesus coming in history. Today, we wait for Jesus coming to us personally as we assemble and pray. During Mass, we wait for Jesus to come to us in the proclamation of the scriptures. We wait for Jesus coming to us in Holy Communion and all the sacraments. We wait for Jesus coming for each of us at the end of our lives. We wait for Jesus coming in glory at the end of time.
Then after more than three weeks of quiet, reflective waiting, we are ready to shout: “Joy to the World!” and “O Come, All Ye Faithful!” and “Hark, the Herald Angels Sing!”