@FinUK, you could use Joe Workman’s TARDIS Stack to show the promo’s on your home page, when clicked then could go to the store and auto fill in a promo code maybe.
What TARDIS does is allow you to set up dates and times materials show/hide on your website, your always on banner could be advent calendar banner image, the next slide could be the days promotion with a promo code, you would make a slide for each day of the advent calendar and tell TARDIS when to show/hide each.
There are other stacks out there that do similar like seyDesign TimeRelease (PHP), CosCulture’s Scheduled Publisher, that’s just at a quick glance through the Stacks section of Realmac.
If you’re wanting to do it right inside RapidCart, I do not know if possible. You would have to tell people RapidWeaver Version, Stacks Version and RapidCart Version for further assistance.
Hope it helps a little