Full explanation in this thread.
Timing would be tight if we go the physical print route but it would essentially be
Competition close 30 November, November winner selected 22:00 December 2nd, artwork sent off to printer by 23:30 that night.
10-14 day turn around at printer, calendars back by 16th December.
Address and postage labels preprinted so just a matter of stuffing envelopes and delivering to Post Office by 17th. Should be with customers just before or just after Christmas.