In one way, it begins today (with the Christmas novena); liturgically, the Christmas countdown starts tomorrow. (I suppose the liturgy just counts novenas differently.) Beginning December 17, the Church brings out Advent's "big guns": at Mass, we'll start to hear the most dramatic Messianic prophecies, linked with matching passages from the Gospels (mostly the "infancy narratives" of Matthew and Luke). The Liturgy of the Hours shifts, too. No more generic Advent antiphons and Scripture verses, but carefully set texts that (in some cases) have been prayed on that date for over a thousand years. Foremost of these are the "O Antiphons," a liturgical acrostic that, by Christmas Eve, will have spelled out (in Latin, of course) the cryptic message "Ero cras" ("Tomorrow I shall be").
It's a blessing to have a somewhat full Advent season so that the fourth candle on the wreath gets a bit exercised, too, don't you think?