When I did my challenge towards the end of July, you could choose any 3 month period for a challenge. The only restriction I was given was that flights that were already posted for that particular month would not count - for example, if I signed up for a Dec/Jan/Feb challenge the second week of Dec, any flights I made in December that I'd already gotten credit for from the first week of Dec wouldn't count.

My advice - call someone at the DM service center and ask for a challenge in Dec/Jan/Feb sooner rather than later, just in case you do have early Dec flights.

happy Trails!