Shakespeare's "Twelfth Night" is the only play of his which has an alternative title, namely "What You Will."
'Twelfth Night' is considered to be a reference to the 'Epiphany' which happened to fall on January 6th the twelfth night after Christmas. Although the first recorded performance of the play is 2nd February 1602, it is quite possible that the play was first staged on January 6th and so Shakespeare gave it the title "Twelfth Night." January 6th in Shakespeare's time was a holiday and the festival was celebrated with lot of riotous fun activities which involved turning the social order topsy turvy not unlike what happens in the main play.
The sub title "What you Will" translates roughly into the modern 'whatever.' By giving it this sub title, Shakespeare indulges the audience in this festival mood of merry making almost saying that they are free to call his play by whatever title they want.
It has been a matter of conjecture that Twelfth Night was one of four plays the performance of which before Queen Elizabeth in the Christmas festivities of 1601-02 is recorded. In John Manningham's Diary, the entry relating to Twelfth Night occurs under the date of February 2, 1602. In any case, the title refers to the festival of Epiphany on January 6, and the atmosphere of fun and festivity associated with its observance. The play has a court fool Feste, singing a number of songs and making wonderfully witty repartees. Besides, there are elements of comic mischief, disguises, reference to 'cakes and ale'.
The play bears a sub-title 'What You Will' which is also in consonance with the theme & atmosphere of the play. Shakespeare was, as it were, allowing his audience absolute liberty to name or assess the play. The occasion and mood of the festivities of Twelfth Night perhaps demanded so.