Masks suggest the deceptive appearance we present to others in waking life.
If you see a pair of masks, this can symbolize your inherent duality—in Jungian terms, the opposed forces of light and shadow that exist within each one of us. Dreams that feature costumes, fancy dress or drag suggest that you are attempting to disguise some aspect of your personality. Dreams of transvestism—deliberately dressing in clothes of the opposite sex—are unlikely to be about sexual preference but more about the need to balance the masculine and feminine aspects of your personality.
If you are wearing a historical costume in your dream, either for a play or as normal dress, then perhaps your image needs updating or a part of you is disguising your real self.
If you come face to face with someone in your dream and they are wearing a mask, your dreaming mind is sending you a powerful message that you believe they are wearing a false mask in waking life.
[1]