I was noticing how many Christmas carols and hymns assume that "heavenly hosts sing Alleluia" on Christmas.
Truth to tell, "Alleluia" is not the angelic Christmas song. According to the Gospel of Luke, the "heavenly host" was singing "Glory to God in the highest, and on earth peace." It's just that "alleluia" fits into a melodic line a bit more easily than such a long and complete message. (Our Founder latched onto the Christmas message and made it the whole focus of our mission, saying that the angels' song was really about what Jesus came to do, and so we are to continue the selfsame mission.)
Does that mean that "Alleluia" should be banned from Christmas? Not so! Alleluia is the Church's Easter song: when we sing it at Christmas, we are implicitly invoking the child's future resurrection from the dead. When we put it in the mouths of the heavenly hosts at Christmas, it is a kind of angelic prophecy of how the infant will bring about "Glory to God in the highest, and peace on earth." It is anticipated praise, so to speak.
Now, as for singing... My favorite Christmas carol is "It Came Upon the Midnight Clear" (interestingly, one of the few songs that puts the right Christmas words in the angels' mouths). What is your not-to-be-missed absolutely favorite carol?