If you’re watching the liturgical calendar in the Lectionary, Missal, or Liturgy of the Hours, you’ll notice that the enumeration for the weekdays suddenly switches today from “X day of the Nth Week of Advent” to “December 17,” “December 18,” etc. The weekdays from December 17 to 24 are reckoned this way because the liturgy in these days is determined primarily by the exact number of days until Christmas, instead of the relation to the Sundays of Advent.
In other words, we’ve begun the countdown to Christmas.
As dawn nears and the anticipation builds, the themes of the Lectionary readings move from the preaching of John and the end times to the specific runup to the birth of the Lord, as well as prophecies and foreshadowings of his coming. We go straight through the genealogy and birth of Christ in the first chapter of Matthew, and then through the circumstances of John’s birth and the Annunciation to Mary in the first chapter of Luke. On December 24, this leaves us hanging right before Luke’s Nativity, which we’ll read at Midnight Mass.
Meanwhile, at Vespers, the Church begins to chant the great O Antiphons for the Magnificat. These antiphons are the basis for the verses of the Advent hymn “O Come, O Come Emmanuel,” addressing Christ by seven different titles and pleading for him to come: “O Sapientia” (Wisdom), “O Adonai” (Lord), “O Radix Iesse” (Root of Jesse), “O Clavis David” (Key of David), “O Oriens” (Dawn), “O Rex Gentium” (King of the Nations), and “O Emmanuel.”
When you arrange the first letters of these titles in reverse, you discover that they’ve been cleverly arranged to spell out Christ’s Christmas Eve response to our incessant cries of “Veni!”: “Ero cras,” he says, or “I come tomorrow!”