As a busy working mum I have always liked to have everything sorted for Christmas by the end of November, as December is(was) filled up with school Christmas plays, and lots of weekend Christmas activities. Anyone who says keep it in December hasn't got a clue as to how much there is to do for families with small children, and if things aren't sorted by late November then that leads to enormous stress of trying to fit it all in.
My husband always says keep Christmas in December and says all Christmas shopping should be done in December, think you can tell he only has one present to buy (which he often does on Christmas Eve) as opposed to stockings, tree presents for kids and presents for the rest of the family, neighbours kids (we go there on xmas eve) babysitters, teachers etc etc not to mention cards to write etc
As for family arrangements, when we were on talking terms with my brother I liked to try and sort this out in October/November, as I had other relatives to fit in as well, and he and his girlfriend just didn't see the problem, so we had OH's family and brother's girlfriend's family phoning to see what we were doing so everything could fit in, one advantage of not seeing my brother anymore.
When I had time to make one, I made the Christmas cake in October -so I could feed it.