There was really bad snow in early January 2010. The roads in and out of Siurana were closed for 3-4 days and the main road up to cornadella from the coast was also closed. Probably the worst snow I've ever seen and that includes aviemore and glencoe in winter.
Was out in mid December past and although it was pretty cold, the weather was perfect and have been out before and the weather was quite mild around this time.
I'd say safest bet would be go early December or you could head to Costa Blanca if it snows.