Wouldn't bother personally, Christmas is a time when people are most likely to splash cash so I'd take the opportunity to capitalise. Plus, it's stressful enough as it is!
At a push I'd create 5-10 Christmassy designs & offer them all at a set price (£5 or so less than normal)
Also I'd give a £5 give voucher for use in Jan if I was feeling generous mwuhaha