There are several threads that have mentioned upcoming events around Switzerland.

For convenience, I have started this thread to keep the key ones together.

To start with, of course, the Swiss Tourism website: My Switzerland
This can be sorted by area, date, type.

I found this one today and it has a quite comprehensive listing.
Frommer's What's On When.

you can also have a look to the Tourism Office website of the town you live in/or going to visit.

for Geneva, you have also http://www.agendadegeneve.ch which is quite nice.

Other source of information are the local papers.

Specific for Zurich, and really good - includes accommodation, restaurants, activities, tours, cultural events, etc.

http://www.zuerich.com/en/Booking.html