Well, I'm not sure how "official" it is but we start on Christmas Day. We started celebrating the 12 days of Christmas because I found it helped with the post-holiday let-down that can sometimes happen (not just to the kids, but to me, too, you know?)
We have a little Nature Corner and for Christmas it has a tiny tree and we put our Advent calendar there, too. Christmas Day evening I usually take down the Advent calendar and replace it with a little something - a little present or craft or piece of candy, etc. It is a nice surprise at the end of the day when you think all the presents are done to find a little something extra.
Last year I bought white lunch sacks and decorated them with Christmas stickers and each sack was numbered 1 -12 and set them under the tree in the Nature Corner and each day my daughter got to pick the next number. I just put small toys or candy in each one.
ackage: This year we're on a super tight budget, so I'm using more free sources from the internet (paper dolls, coloring pages, foldable toys, etc) and homemade presents (like felt covered barretts, handknit toys, one day is a coupon for baking cookies together)
Each day after Christmas a few more decorations come down around the house as we return the house to normal in time for Plough Monday and I think these little treats help with the transition, plus it is fun to come up with fun, inexpensive ideas and crafts. We count up to Christmas during Advent, so I have always felt that it helps wind down the holiday season (especially for my 6 year old) to count it down afterwards, you know? Plus, it helps that holiday magic hang around a little longer :haha: