In reply to Henry Connor:
I've climbed a few times at Buoux at Christmas / New Year. It was fine - in fact, perfect, during the day without being too hot, although it does get dark quite early (and quickly!) and it's freezing once the sun goes down. I've generally had worse weather at Easter (wet).
I recall doing Pilier des Fourmis as jolly party of 4 at New Year and a great night in the auberge just below the route.