This is "must-watch cinema" if ever there was.
I also understand this one is in the public domain, and is available from a number of sites for those who don't want to go to the shops so close to Christmas. The Internet Archive has it in a number of formats, there for the taking.
http://www.archive.org/details/santa...s_the_martians
I think it is also to be found on a number of the public domain/legal torrent sites.