Amy,
In general, I send Christmas stuff during the first two weeks of November (I think the official guideline is to ship before Thanksgiving). If you're shipping things to border outposts or groups prone to extended missions, or if you're shipping cards for them to send back to their families, you might want to give things a little more time.
Outposts that get infrequent mail deliveries often seem to have their mail processed on a First-In-Last-Out basis which can cause significant delays with some packages. Mail coming back is not given high priority, so that will be even slower.
I agree that ambitious projects require more time also.