We've only ever been in December or November - I hate going when it's really crowded!
The last three times we have been early to mid December to enjoy the decorations - but boy, can it get COLD. Our very first visit was in early November which, looking back on, the weather seemed positively balmy by comparison!!
Anyway, although my DH and I don't really mind it getting so cold as we just wear loads of layers, now that we have our DD to think of I'm planning my next trip for November again. DLP put their christmas decorations up so early we can enjoy those and, hopefully, some very slightly warmer weather!!