just heard on the radio this morning: it was a live riff on "12 days of christmas" by what sounded like a classically trained male choir.
They started mixing up the lines around the 4th verse, and then working in lyrics from other songs (eg "... two turtle doves/and a Rudolph the Red Nosed Reindeer");
at the end they broke into "I have a dreidle" on the 12th verse, and then worked in a very good "Christmas down in Africa" to finish it off.
Audience was enjoying themselves and it was very well done.
Must know who did it.
Found it!
Slowpokes.
Straight No Chaser
And, yes, they are on iTunes!
That was awesome! Hadn't seen it before - they did an excellent job. Thanks for finding it. :)