What points signify that Twelfth Night is a festive comedy?
"Twelfth Night", the title of this play, refers to the feast of the Epiphany on 6th January, when the wise men were supposed to have given Jesus his gifts. The feast of "Twelfth Night" also had another meaning - it was the last "bash" before the Christmas tree gets taken down and therefore a festival where people were expected to enjoy themselves by drinking, eating and having lots of raucous fun.
So, the title is the play's clearest indication that this is a festive comedy. Also, you might like to look at Act II Scene 3 where Sir Toby Belch and Sir Andrew Augecheek and Feste have a bit of a late-night celebration. Sir Toby at one stage starts singing "On the twelfth day of Christmas". Indeed, you can see Sir Toby Belch as being the spirit of the Twelfth Night celebrations in the play - he refuses to stop partying and drinks and parties to excess. Indeed, he says:
But shall we make the welkin dance indeed? shall we
rouse the night owl in catch that will draw three
souls out of one weaver? Shall we do that?
He therefore is in confrontation with Malvolio, who represents the spirit of Puritanism, decorum and sensibility. Interestingly, the play ends with the end of the licensed excess, with the formalisation of the three unions, and therefore an end to the chaos of the "Twelfth Night" festival.