Father Christmas, Santa Claus, Old Saint Nick. Call him whatever you like, he’s still the same fellow. The Doctor knows him by a different name: Jeff.
They hang out now and then, sometimes just the two of them, sometimes with others. There was that time in 1952, at Sinatra’s hunting lodge; that was fun.
“So, Jeff, how’s tricks?”
Santa Claus smiles through his beard. “Oh, you know me, Doc. Can’t complain!” Not that he would anyway, he’s never been the complaining sort. “How about you?”
“Oh, this and that.” They sip their tea companionably. The Doctor loves these little chats.