In Shakespeare’s Romeo and Juliet, Romeo wears a mask to enter the Capulets’ feast.
Romeo and his fellow Montagues attend the Capulet celebration without being invited. Because the two families are enemies, they disguise themselves so they can enter safely and avoid immediate recognition. Romeo’s mask allows him to join the feast, where he first sees Juliet.
The disguise does not conceal his identity for long. Tybalt recognizes Romeo’s voice and wants to attack him, but Capulet stops Tybalt because he does not want violence at the celebration.
Modern productions often stage this moment with elaborate costumes, so the exact visual design varies. The text’s essential point is that Romeo attends masked, not that he wears a particular historical costume or uniform.