I'll be in Hong Kong over the Christmas holiday (Dec 20-28). I will spend part of that time (not sure when) in Macau.
What sorts of closures should I expect over this period?
Only possibly on the 23rd, most likely not even then.
First of all, shops and malls are usually open on Public Holidays in Asia. Second of all, there are some public holidays during the 20-28th December, but they will not affect tourists. Malls and Shops will be open, except maybe for a couple of privately run restaurants or shops. However, since Macau lives to a huge extent from tourism, the vast majority of places will be open. If you have a special place you want to visit, you might want to check online whether that place is open, and then book a table as well.