Traditionally, Twelfth Night is the last of the 12 days of Christmas, January 5, and also known as the Eve of Epiphany.

According to Wikipedia:

In medieval and Tudor England, Candlemas traditionally marked the end of the Christmas season, although later, Twelfth Night came to signal the end of Christmastide, with a new but related season of Epiphanytide running until Candlemas. A popular Twelfth Night tradition was to have a bean and pea hidden inside a Twelfth-night cake; the “man who finds the bean in his slice of cake becomes King for the night while the lady who finds a pea in her slice of cake becomes Queen for the night.” Following this selection, Twelfth Night parties would continue and would include the singing of Christmas carols, as well as feasting.

[. . .] Modern American Carnival traditions are seen across former French colonies, most notably in New Orleans and Mobile. In the mid-twentieth century, friends gathered for weekly king cake parties. Whoever got the slice with the “king”, usually in the form of a miniature baby doll (symbolic of the Christ Child, “Christ the King”), hosted the next week’s party. Traditionally, this was a bean for the king and a pea for the queen. 

[. . .] In some countries, Twelfth Night and Epiphany mark the start of the Carnival season, which lasts through Mardi Gras Day.

Also, Twelfth Night, or What You Will, is a romantic comedy by William Shakespeare that is believed to have been written around 1601–1602 as a Twelfth Night entertainment for the close of the Christmas season. The play centers on the twins Viola and Sebastian, who are separated in a shipwreck. Viola (who is disguised as Cesario) falls in love with the Duke Orsino, who in turn is in love with Countess Olivia. Upon meeting Viola, Countess Olivia falls in love with her thinking she is a man.

Maclise, Daniel; Scene from ‘Twelfth Night’ (‘Malvolio and the Countess’); Tate