Marye is correct - for the best traffic result, your seasonal articles need to be up well ahead of the season itself. If you publish Christmas related articles now, you'll still see a small bump, but nothing like you would be seeing if you had published your Christmas articles by September or October (or even sooner with Christmas, since it seems like the season starts earlier and earlier all the time).
But posting Christmas articles now isn't a total loss, as long as the articles you're writing are evergreen. 'Best children's toys of 2011' has a short shelf life, 'Best children's toys of all time' or 'Best classic children's toys' will be searched for pretty much every year. It's possible to build a stable of seasonally themed articles that will attract readers every year if you think in terms of providing information that will always be wanted during a particular season.