Twelfth night is a holiday marked by some branches of Christianity, marking the 12th and final night of the Christmas season, namely the eve before twelfth day[?] or January 6, the Epiphany celebration to commemorate the adoration of the Magi[?]. After Twelfth Night the Carnival season starts.
The play is so named after the Twelfth Night holiday. Since it is difficult to see any allusion to Epiphany in the play, many suspect the play was written to be performed during Twelfth Night celebrations.
Shakespeare's play, like so many of his comedies, centres on mistaken identity. The leading character, Viola, is shipwrecked in the opening scenes, and loses contact with her twin brother, Sebastian, whom she believes dead. Masquerading as a boy, she enters the service of Duke Orsino, who is in love with the bereaved Olivia, and uses Viola as an intermediary.
Olivia, believing Viola to be male, falls in love with her. Viola, of course, falls in love with the Duke. When Sebastian arrives on the scene, confusion ensues. Befitting a comedy, the play ends in marriage with the Duke marrying Viola and Olivia marrying Sebastian.
Much of the play is taken up with the comic sub-plot, involving Olivia's uncle, Sir Toby Belch, and her pompous steward, Malvolio. Other major characters are Sir Toby's friend, Sir Andrew Aguecheek, and the clown, Feste.
Although this is one of Shakespeare's most popular and funniest comedies, it has a dark side, as the behaviour of Sir Toby and Feste towards Malvolio becomes quite sadistic towards the end.