Shops will be allowed to open every Sunday in December, so if you need to do any Xmas shopping, you will have more chances to do so!
However, the fact that shops are allowed to open, does not necessarily mean they will. Shops at the Porte de Namur mall, for instance, already opened last weekend, but will not be open next weekend (or at least according to some of the signs they posted… then again, this is Belgium so better check before going or not going!)
More information on the Brussels.be site. You can also check the sites of two major chopping malls – City2 and Woluwe Shopping – before going, as these are god indicators of when most other shops will open. They usually put news about their special opening hours a day or two before the Sunday opening.