Time for me to chime in.
So, throughout the year (this year), I very rarely even looked at my Christmas site. Then, in September, I gave it a quick once over, and did the following...
1. Added a half dozen images on one internal page (of a site that has several hundred pages).
2. Checked over my affiliate links to make sure they were still valid links. Removed one or two that were not.
Then, in late November, I made one other change - I changed the adsense ads from one size to another - on two pages (the home page and one interior page).
I did not actively seek any backlinks throughout the entire year. I also didn't really notice any new ones pop up, but I suppose there might have been a handful of unasked-for backlinks that came about during the year.
So, more or less, I did almost nothing to the site all year. Very, very minor changes is all.
From January to September, I had semi-decent rankings - mostly in the 20's range.
In September, bumped up to top 10's and stayed that way till today, when I dropped down to 90's and below.
Last year, when I dropped down, the rankings came back within a few weeks to the 20's. Will be interesting to see if the same pattern follows.
Yahoo and MSN - top 5 all year long - and still top 5.
Traffic: maybe 10 users per day average until the season started. Once October came along, bumped up to 1000/day. Then, 10,000/day in November and 20,000/day in December (averages). Back down to 1,000/day right after Christmas, and will probably be down to 10/day within the next month.
So, there's the facts for my site. None of my other holiday sites seem to suffer the same problem, but it may have something to do with the Christmas season. Perhaps Google does something special just for that season. Dunno.