like normal people they can take vacations of varying length throughout the year . Most people go on holiday over Christmas/New Year until mid-January, though. Like with most government institutions, a core staff is available for urgent cases and emergencies. Think of your own summer and Christmas vacations rolled up in one and expect about 3 weeks (= my estimate) delay.
I called DIAC and they say technically they only close on Christmas-25/12, Boxing day-26/12 and the Friday 27/12, then New Year 1/1. But the reality.. I don't know. Put 2-3 weeks extra for the holiday mood.