TWELFTH NIGHT

Shakespeare wrote Twelfth Night between 1599 and late 1601. It was originally published in the First Folio  in 1623.

Twelfth Night is the night of Epiphany, 6 January, or, the twelfth night after Christmas. This holiday signalled the end of Christmas merry-making and, more importantly, commemorated the Magi. It is likely that Shakespeare wrote the play to celebrate this occasion and thus gave it this specific title.
Here  you can find the full text of the play.  

Various critics divide Twelfth Night into various types of plots and/or subplots.  Regardless of the exact number of plots and subplots, however, the main thing is that they are all woven together with immense skill to ultimately compose a single pattern or tapestry. There is, first, the group centering around the ducal nobility of Illyria: this group consists of Duke Orsino and his attendants, who open the play, and the Countess Olivia, who is the main topic of discussion of the opening scene. Then there is the group of shipwrecked personages centering on Viola and Sebastian, the twins, and their friends, Viola's sea captain who fades from the action, and, more important, Antonio, who plays a significant role later in the comedy. Both Viola and Sebastian are, of course, later absorbed into the nobility of Illyria. Then there is the merry group of pranksters, gullers, and tricksters, led by Sir Toby Belch and Maria; this group also includes Sir Andrew Aguecheek (who is included because his income supports the other members of this group), Fabian, and Feste, the Clown. Through Feste, all of the groups are connected by his free movement from one group to another as he is equally at home singing for Duke Orsino, or proving Lady Olivia to be a fool for so excessively mourning for her brother, or in planning a trick with Sir Toby. Then outside of all of these groups stands Malvolio, Lady Olivia's puritanical steward. His colossal vanity and egotism get between him and everything that he sees and does. Thus, he has already gotten on the wrong side of Maria, Feste, and Sir Toby, and the plot involving their determination to take their revenge upon him provides the best humor of the play.

Malvolio is socially and sexually ambitious; Maria realizes this and writes a letter purporting to come from the Countess Olivia, making Malvolio believe that his lady is in love with him and wishes to marry him; the letter also asks him to be firm and obstinate with her uncle, Sir Toby, to be arrogant to the other servants, and to dress in yellow stockings and go cross-gartered, and to smile all the time when he is near her. Malvolio finds the letter on the garden path and falls for the trick as he is watched gleefully by the group led by Maria and Sir Toby.

Viola disguises herself as a boy in order to protect herself and to obtain employment by Duke Orsino and quickly finds her way (as Cesario, the youth) into his favor; she is then sent to woo the Countess Olivia, much against Viola's will, for she has fallen in love with Count Orsino herself. Countess Olivia, who cannot love Duke Orsino, falls immediately in love with the messenger, Cesario, thus creating an amusing triangle which produces several complications. The arrival of Viola's twin brother, Sebastian (previously presumed drowned), sorts everything out matrimonially. Sebastian marries Olivia, Orsino marries Viola, and Sir Toby marries Maria for having played such an excellent trick on Malvolio.

This is one of Shakespeare's most popular, lightest, and most musical of all his comedies, and its staging continues to delight audiences all over the world.

from  http://www.cliffsnotes.com/literature/t/twelfth-night/play-summary

Here you can find some very useful material to revise the play.