With under three months to Christmas, it's time to get cracking if you plan to launch a DM campaign sooner. Unless there is a compelling reason to do so, I recommend going no later than the second week of December. After then people are winding down and in holiday mode.
If you are planning a January send (especially to businesses), it's best to hold off until the last week of January when most people and their colleagues are back from leave. Going earlier does however have the advantage of people being less time poor than they will likely be in a few months when their workload increases.
With overall mail volumes decreasing, there is less clutter in the letterbox than in past years. This presents an opportunity to those businesses wanting to go down the traditional path. Mailing lists tend to offer better coverage of a market than email or telemarketing lists so why not give snail mail another chance if you've been avoiding it?
I believe a well presented, strong offer which is targeted correctly and personalised will make more of an impression today than since the late 1990s when personalised DM really took off in Australia.
Whatever type of campaign you have in mind, get in early with your planning. If you are finding it hard to find the time, a mail-out can generally be turnaround within a week. Email lists generally have a one to two week wait to schedule a broadcast date while telemarketing lists can generally be sent to you within 24 hours of ordering.