I don't have any experience in disputing during X-mas but I would say towards the middle of the month... The CRA would get the letter around the 20-24th (hopefully) then they would be off for Christmas, back for a week (maybe, unless it is a small agency) then off again for New Year's (maybe) and then a couple weeks after that is Martin Luther King.
All in all, even if the CRA stays open during most of the holidays, most times the OCs do not.
Good luck with your disputes!