How do I make individual posts display a different theme?

I want to find a way to create a new theme each week and have the theme stick to the individual posts for that week. (Think The Bleat when Lileks was using HTML only.) I’m thinking the theme would have to be linked to the time stamp for the post somehow but beyond that I am lost. In fact I’m so lost I’m not sure I’m asking the right question.
Any ideas?

To pre-empt at least one question I am not looking to change themes with individual pages, only with posts.

Thanks
Jon Jackson