Mr. Foolish

Twelfth Night is more than simply a great play it's a Christian holiday that began in Tudor England. The Oxford English Dictionary states that it's "the evening of the fifth of January, preceding Twelfth Day, the eve of the Epiphany, formerly the last day of the Christmas festivities and observed as a time of merrymaking". (This was taken from Wikipedia)

Moreover the festival celebrates the Lord of Misrule who was known for causing disorder. This falls right in line with Shakespeare's comedy as the characters of Maria, Sir Toby and Sir Andrew do nothing but cause much disorder. In fact even the more serious character of Viola causes quite a disruption. In fact, in some cultures the festival of 'Twelfth Night' marks the beginning of carnivale season, or Mardi Gras. The end of the festival marks the coming of the Epiphany. This relates back to Twelfth Night to the point that the characters are marked with an epiphany at the end of the story (the end of the Twelfth Night) as revelations about the characters disguises are revealed.

Although written a couple of centuries later, Victor Hugo's masterpiece The Hunchback of Notre Dame takes place during the celebration of the Epiphany. Although the book was written as a testament by Hugo to maintain the original structure of the cathderal there is also a major undertone to the novel not about mistaken identities as in Shakespeare, but about mistaken appearances. Quasimodo who would be thought of as nothing more than a mamed imbecile is reality the most humane of any character in the novel. This uniquely parallels Viola who, although she is physically disguised, she remains the most honest to her heart, never does she consider courting anyone besides Orsino.

It's typical of Shakespeare to inculcate his messages. Here he has the main story-line about Viola's disguise, but then there is Maria's disguised letter, Antonio -a pirate- representing himself as a good friend, Olivia is supposed to be mourning her brother's death but instead is out to get Cesario. Therefore it's extremely likely that Shakespeare chose to name his play after the Twelfth Night holiday in order to represent all of the topsy-turvy during the festival and the play itself.