The title of this play refers to a holiday during the time when people could feast and act in ways that were opposite to their usual personalities. It was a way to let loose of general worries and daily routines and adopt, for a time, an alternate persona.
The characters in this play seem to have adopted this attitude even though they do not realize it. Viola adopts the persona of her twin and pretends to be a man, while her actual twin. Olivia, who spurns the duke in favor of Viola/Cesario, parallels the Duke's ultimate attraction to Viola. The world, like the intent of the holiday, is turned upside down. Of course, things settle down, and two, happy couples emerge at the end of the play, devoid of disguise.
The idea of donning a disguise or playing a role is attractive as a way of forgetting one's daily life. This holiday, and the play, celebrates that.