Theseus has planned a revel because he is marrying Hippolyta, formerly queen of the Amazons. He is in love with her and excited to marry her. He says at the beginning of the play that he wishes the wedding, which is almost upon them, could be even sooner.
Hippolyta is not quite so keen to get married and is willing to wait the short time that must elapse. Theseus kidnapped her in battle, and initially she was not in love with him. Gradually, though, he has managed to woo her so that she is gracious in accepting the idea of him as her husband.
A wedding is a fitting frame for a play about love in its many variations. Despite all the many mishaps the characters encounter during the course of a Midsummer's Eve, a joyous wedding celebration is a fitting end to this play.