Traditionally, people thought of Christmas as being a 12 day festival.
The 12 days of Christmas began on what is now Boxing Day, the day after Christmas. They were the time for merry-making, which is why they began after the religious observances of Christmas were over. Six of the days fall in the year that is ending; the other six fall in the year that is coming.
Most work would stop during this time, giving everyone a break.
Twelfth Night actually occurred on the evening of the 11th day, the 5th of January — so it was the night before the twelfth day. The actual twelfth day of Christmas is called Epiphany.
Twelfth Night used to be a great deal of fun. Parties would be held, practical jokes played, and there would be feasting, singing, charades and dancing.
A Twelfth Night Cake was an integral part of the evening.