it's the title of a Shakespeare play, but it's also a legit holiday that rolls around this weekend (there's apparently some debate over whether the Twelfth Night falls on Jan. 5 or 6, which is problematic in that you may reap bad mojo if you leave your holiday decor up past Twelfth Night). From what I can glean, it's basically a festival to celebrate the Epiphany (in Christianity), to mark the end of the 12 Days of Christmas, and generally, to give people a religious excuse to eat, drink, and be merry for just a few more days into the New Year.
I was trolling ye olde Wikipedia looking for info, fully intent on writing up a little primer, when I came across author Sarah M. Eden's informative blog post about it.
She summed up all the nitty gritty far better than I, an oblivious Yankee, ever could. Bottom line, I'm all for any holiday that manages to milk a few more days of holiday indulgence, though with all that king cake and mulled wine, it seems certain to mess with many people's healthy New Year's Resolutions.