While I suspect O Come , O come Emmanuel may be the best known Advent song in the world, you may not understand its place in the liturgy.
Beginning on December 17th as part of Evening prayer we sing one of the O Antiphons to accompany the Magnificat. Each provides an Old Testament linkage to Christ.
Today's is the fourth: O Key of David
For us Christ is that key which opened the gates of heaven, and made it possible for humans to enter where only God and angels had previously been. In many ways however Christ is not simply the key to heaven, but the universal key. He is the key that can open hearts, open minds. He is the key to forgiveness and reconciliation.
A part of the stress of the holidays is that we are often forced to come together with people, sometimes even family members, who we may not honestly like. Rather than gritting our teeth and putting on the fake smile, perhaps the best thing we can do is pray and invite that key of David, to change us, to open our hearts to see and love whatever good there is. And there is always some good in everyone. Pray for the key of David to lock the chains of old angers and resentments that keep us bound, that we may know the freedom to which we are called.
This way every "Merry Christmas" that we utter can be sincere and come from the very depths of our heart, carried by God's grace.