I have examined the conditions for participation in an opera masquerade. Theoretical support has been found in Michail Bachtin, Richard Schechner and Judith Butler. The main study object has been a masquerade organised by the workers of The Royal Opera of Stockholm in 1925. The masquerade is examined as a social practise with distinct aesthetic framing. The analysis shows that the opera masquerade share some characteristics with the medieval carnival culture as described by Bachtin, but differ in many ways as well. It is easy to place the opera masquerade within the concept performance as discussed by Schechner, since it is framed in time, space and make use of certain social agreements. A look at the architecture shows that physical and social border between the stage and the auditorium had to be dissolved to let opera viewers become opera masquerade participants. Using Butler I discuss how the masquerade could challenge as well as preserve traditional gender structures.