We have stumbled upon these events which attract huge numbers of people on four separate trips to Switzerland. In the case of one, it was a nice surprise and we enjoyed the scene. The other was "interesting" but a bit of a headache for train travel ( we did not attend the event itself as we were simply passing through on our way to or from the Zürich airport.). I will let you guess which is which.
I thought I would mention them here for the benefit of others who plan to visit Switzerland this summer or next. You might want to make plans to attend, or avoid the area entirely.
Zürich Street Parade:
https://www.zuerich.com/en/visit/street-parade
http://www.streetparade.com/en/
August 11 this summer. I did not check on 2019 but it generally is the second Saturday in August. A million or so people converge on Zürich and the trains and main train station are packed and chaotic. You will hear loud techno music ( in the station and out on the street) and see lots of people dressed in feathers, paint, tinfoil, and little else.
Zürichfest:
https://www.zuerich.com/en/visit/zueri-faescht
This is when Zürich throws a great party, with music (all kinds), a street fair with food booths stretching more than a kilometer along the riverfront, airshows ( with helicopters) and other festive events. It only occurs every three years, and the next is the first weekend in July, 2019.