In reply to Yanis Nayu:
Usually November, yeah, though It Depends On The Weather.
November's -usually- mostly just a bit miserable, with little in the way of decent Autumn or Winter weather, but the odd time it surprises with some proper snow or a warm sunny spell.
December's a close runner for the position, for the same reasons, though there's more chance of getting decent Winter days on the hills.
Both of these months can quite often bring a beautiful temperature inversion, with gorgeous sunny Winter weather on the snow-covered hills, above the clouds, though.
When it comes to it, any month can be great or awful, or more likely a mix of these. But November and December are the most likely to be bland and miserable, I think.