In Romeo and Juliet, the Capulets host a party where the attendees wear masks.
Well, the masks at first glance hide the identities of the people wearing them. This is to possibly facilitate the mingling of the crowd of young guests without anyone feeling ostracized or judged because of their true identity.
Additionally, the masks were worn by the young party-goers to contribute to the theatrical effect of the party. Of note is the fact that Hamlet has to wear a mask because he is a Montague and he enters the house of his family's sworn enemy, the Capulet family.