"The Rudolfina Redoute is the oldest, most traditional ball organized by a student fraternity. Of the many Redoutes that once took place, the Rudolfina Redoute is the only one that still survives today. Every year on Carnival Monday ("Rosenmontag"), guests from all over the world come to the Vienna Hofburg for the Rudolfina Redoute. The Rudolfina Redoute dates back to the Austro-Hungarian empire. As the last large ball before Lent, the Rudolfina Redoute is a glorious end to the Viennese Carnival season."
See here: http://www.rudolfina-redoute.at/en/history/.