Why December 20th? I wouldnt expect much to be done over the holiday period! I would however pay your fees a few weeks before you mail your application..seems to help speed up the process a bit
Holidays don't appear to affect the CPC-M processing time for spousal sponsorships much. It fluctuates but except for earlier this year (when I seem to recall reading they reassigned a bunch of staff to deal with the Haiti aftermath) it stays around a month. If you put in your application mid December, it will hit the front of the line mid January anyway, long after the holidays.