It's the only midnight Mass in the Roman Missal, and it's tonight. Some people would never miss Christmas Midnight Mass, even though it's (cough, cough) the only Mass they attend all year. But why is it precisely "midnight" Mass? (It's still the "midnight" Mass readings and prayers even if the Mass is anticipated, as ours will be tonight at Mount Carmel.) Is there something about midnight and Christmas, or is midnight just a particularly romantic hour for a religious observance?
We know about the "night" part of Christmas. Luke makes it pretty clear that Jesus was born at night: there were, after all, shepherds "abiding in the fields, keeping watch over their flocks by night."
But that's not all.
The book of Wisdom, in a poetic meditation on the Exodus, proclaimed, "For when peaceful stillness compassed everything and the night in its swift course was half spent, Your all-powerful word from heaven's royal throne bounded, a fierce warrior, into the doomed land...' (Wis. 18: 14-15). So a dramatic, saving intervention of God took place when night was "half-spent" (ergo, midnight) and God's "all-powerful word" (the Word Incarnate!) descended from the royal throne.
Midnight Mass is the Church's way of saying that this passage was more than a meditation on God's action in the past history of Israel: it was a prophecy of an even more dramatic saving intervention by God through his Word.
So, whatever time your Midnight Mass is, may it be the opening of a Christmas full of Jesus!