'Festive comedy' was sub-genre which a critic called Barber established with her book 'Shakespeare's Festive Comedy'. It argues that the spirit of the festival (meaning simply a public or religious holiday) was to turn the normal hierarchies of the social sphere upside down - and that unacceptable behaviour became acceptable for a single day.
We might see in Twelfth Night many thematic clues towards festive comedy: a play in which the main character is a woman dressed as a man, a play which is catalysed by a storm (literally turning everything topsy-turvy), and a play in which normal behaviour is subverted: Olivia comes out of mourning to woo Cesario, and Malvolio (usually unsmiling and severe) wears yellow stockings and smiles to woo Olivia.
Also associated with festive comedies are celebratory drinking and eating ('festive' is a close verbal relation of 'feast') and, when Toby turns on Malvolio, telling him that the 'cakes and ale' will stop simply because he is virtuous, many critics have argued that Toby and Malvolio represent respectively the festive and the puritan. Malvolio is compared to a puritan by Maria: and the puritans, against enjoyment of most kinds and certainly opposed to festive celebrations, were the enemy of having a good time!
In brief then: what are the main points that might make Twelfth Night a festive comedy? Turning social structure on its head, unusual behaviour, and cakes and ale.
Twelfth Night is a rambunctious comedy, perfect for the Elizabethan holiday of Twelfth Night. The drinking and carousing - as well as the confusion with identities - that goes on in the play is what makes it a festive comedy, despite the unhappy subplot of the tricking of Malvolio by Maria, Andrew, and Sir Toby.
Check the link below for important themes within this play which will help you with your question. Good luck!
Festive Comedy takes its sub-generic name from both occasionalism and philosophical content. These comedies are generally performed on festive occasions and deal with festivity as theme or as a philosophical outlook towards life.
Twelfth Night takes its title from a festive occasion on the 12th night since the crucifixion--the night when the body of the Christ would strike back with epiphany.
At the level of content, Shakespeare's play deals with the clash between mock-puritanical melancholy, as exemplified by the character of Malvolio and the festive spirit of Feste, as his name itself suggests.
The masking trope is an essential part of the rituals of festivity.
The attitudes to love as expressed by the love-lorn Duke of Illyria, Orsino and the superficiality of Olivia and Viola's romantic ideals--all relate to a festive conception of love, satirical in its absolute dearth of seriousness and sincerity.
The festivity in Twelfth Night has its political content in terms of a subversive decadent carnivalesque spirit, as highlighted by the low-life figures and their powerful foray into the court life through Malvolio's gulling.
The sub-plot with all its dark undercolours are suggestive of a tragi-comic mode.