Ed Stetzer just posted some stats about the holiday season. One of them noted that in America, the increase in attendance at Christmas time is mostly regular churchgoers attending more often still. I imagine even of the irregular churchgoers who attend at Christmas time, they are a pretty hardhearted lot, in general. Not the type to be easily won over by a nice Christmas event.
If you can truly reach and evangelise some people @ Christmas time, then that's a fantastic thing. If you can, as Graham commented on my previous Grinch post, use Christmas as a service to the community/raising awareness/making contact time, then go for it.
But I am sceptical of the monumental importance and effectiveness of Christmas events.
P.S. This isn't actually a huge hobby horse of mine. But it does make good blog material, don't you think?