After time is called, each group takes turns singing a few lines of one of the Christmas Carols on their list. No group can sing a Carol already sung by another group. If they do – they drop out of the Sing-Off! Keep going until the groups run out of Carols. The last group to sing a Carol is declared the winner.
Keep it fast paced. If the next group in turn hesitates for five seconds – they drop out!
Give Chritmassy prizes or just give them a big clap and make them sing the whole winning Carol!