Ha! Ha! ..I knew this one was coming Ivan!
Anyway I too plump for ver Slade's 'Merry xmas everybody' (spelling?) and Wizzard's 'Wish it could be Xmas...'. Greg Lake's 'I Believe in Father Xmas' also on the MemberD list.
Mind you just heard Wham's 'Last Xmas ' on the radio which managed to touch a soft-spot somewhere..and I realised I know all the words when a colleague asked me to translate the damn thing!
Off-Xmas theme but still inextricably asociated with said Christian fest are PSB's 'Always On My Mind' and The Human League's Don't You Want Me'...(quickly nips over the Xmas party thread..)