I've often pondered the folly of being so gluttonous for several weeks *before* Christmas, and then almost collapsing with fatigue for the real celebration of Christmas' Twelve Days. Our society likes to celebrate first and recover later (something about hangovers comes to mind).
However, the Church reverses the order: first comes preparations, and then comes the celebration. The preparations - such as fasting, prayer, and giving to charity - help us get ready for the "true message" - and celebration of - the Christ-Mass. When we empty ourselves first (rather than after overindulging), then we can truly enjoy the celebration and feasting...and without the guilt-laden exhaustion.
Here's a though-provoking little piece from Fr. Fenton on devouring the Advent Fast. Now I'll have to track down that article in First Things!