Yep it will be mighty busy, heaps of boats arrive on Boxing day and most stay until New Year. The best place to anchor depends on the size of your boat and wether you want to back anchor near the shore (if you have a smaller boat) but there is usually always room in the anchorage area in Thompson Bay, its on your portside (LHS) as you come into Thompson Bay between the Army Jetty and the Natural Jetty, just stay out of the swimming zone marked by floats along the beach beneath the 2 houses on the hill.
Other popular anchorage areas such as Parker Point and Longreach Bay are usually a bit more protected but fill up fast.
Parakeet Bay is another good spot when it is busy but just watch that your anchor doesn't drag, they often do there for some reason.