In Budapest there are forty theatres, seven concert halls and an opera house. Outdoor festivals, concerts and lectures enrich the cultural offer of summer, which are often held in historical buildings. The largest theatre facilities are the Budapest Operetta and Musical Theatre, the József Attila Theatre, the Katona József Theatre, the Madách Theatre, the Hungarian State Opera House, the National Theatre, the Vigadó Concert Hall, Radnóti Miklós Theatre, the Comedy Theatre and the Palace of Arts, known as ''MUPA''. The Budapest Opera Ball is an annual Hungarian society event taking place in the building of the Budapest Opera (''Operaház'') on the last Saturday of the carnival season, usually late February.