Australia just about shuts down and goes to the beach from late December until mid January.
December is sheer madness as everything hits at once - the end of the school year with its speech nights and other presentations, ditto for sporting organizations' end of season celebrations, family preparations for Christmas (shopping for presents and for victuals for the festive season) and for long summer holidays, office Christmas parties (usually several, by the time you have whole-of-company, department, clients', suppliers' etc), and all of this whilst most industries are working flat-chat right up until Christmas dealing with the rush caused by the fact that they are about to close for 2/3/4 weeks!
By contrast, in January you're lucky if a tradesperson or a commercial concern will even answer the phone.
Christmas seems an unnaturally early start to the school summer holidays. From a climatic point of view, many people would prefer the more predictably warm period of mid January to end of February. We often find that the really hot stinking weather (say 38C/100F+, and I'm in one of the more temperate areas) hits just as the kids go back to school at the end of January.
I think that the northern hemisphere gets a better deal out of Christmas. You get a nice little cheer-me-up in the middle of winter when you need it most, all the traditional snow-and-reindeer stuff works better without the blowflies and sunburn, and it isn't complicated by as many other activities.
