December might not be the best time, just like the summer months might not be, in that there's usually quite a few people on vacation. If HR is on vacation your application might not get processed anytime soon, if the person who has to manage you is on vacation HR can pass on the application but it still won't get an OK until that person is back and tells them to invite you.
On the other hand many of the possible applicants might be on vacation too. If you wait for them to be back you'll not end up at the top of the pile probably, so as several already suggested I'd apply whenever you look for a job. Especially if you don't have a job now it's easier to say 'I was looking for a job' when asked what you did when you weren't working than 'I was waiting for the perfect time to apply, that's why I didn't do anything for half a year!'.