Also called "Twelfth night," this is the last day of Christmas and the day before Epiphany. Historically, feasts and evening parties were held on this day through-out villages (and many cultures still hold theses traditions presently). During these parties, roles in society were often reversed, with servants being served by the nobles. Sometimes a coin or dried bean was baked into a rich king's cake, and whoever found the coin or bean in their slice was lord or lady of the evening.
Today, January 5 is still a fantastic excuse for hosting an evening party with friends to celebrate the finale of Christmastide.