B-A festival celebrated during the Middle ages on the 12th night after Christmas
The 12th day is the day after Christmas. It is the day the magi bestowed their gifts upon Jesus. It marks the end of the Christmas holiday celebrations. This time is one of partying and silliness. The play mirrors this with the tricks the characters play on each other, Sir Toby Belch, Sir Andrew Aguecheek, and Feste.