I know that techinically this makes 13 Days of Christmas but what the heck.
This excellent Canadian film from 1978 was directed by Daryl Duke (who directed the Thorn Birds mini-series) and was a remake of a Danish film Think of a Number (1969). This adaptation was written for the screen by a young Curtis Hanson who gone on to direct L.A. Confidential.
Elliott Gould stars as Miles - a bank teller in a shopping mall in Toronto. It's Christmas time and Gould discovers that the bank is going to be robbed. He then comes up with a plan to steal a bunch of money and blame it on the robber - a psychotic Christopher Plummer. When Plummer comes to rob the bank in a Santa Claus suit, Gould gives Plummer some of the dough and keeps the rest for himself. Plummer discovers that he's been taken and a deadly cat and mouse game begins between the two men.
This movie has many twists and turns and to reveal any more of the plot would destroy the fun of watching the movie. This was one of the earliest films from Canada to take advantage of the Canadian government's "Capital Cost Allowance" plans. And it won the Canadian Film Award for best film that year. The cast is stellar and even John Candy has a bit role in it. The film is one of hidden gems that nobody ever really knows about - in fact I can't even find a trailer or a clip of the film but it is finally available on DVD.