Once a year the Vienna Philharmonic Orchestra plays a concert without charging for tickets. The performance takes place in front of Schönbrunn Palace. To get a seat you should arrive rather early or you take a blanket with you for sitting on the lawn of the Gloriette hill.
To get an impression of this concert, this is the one of 2022:
https://community.ricksteves.com/travel-forum/austria/are-you-in-vienna-today-june-16#1935353