I don't know how many of you Zürich folks know about this already, but we attended last month and it was a great event. Basically you pay one entrance price (CHF 25) for an armband that gives you access to transportation and all the participating museums, galleries, etc. located all over Zurich. It's just one time per year but all there were so many places to see that we barely hit 1/2 of the places we planned.
It's not just art venues, the Zürich Zoo, Corbusier house, Botanical Gardens, Paleontology Museum and Tram museum were just a few other venues participating. Everything stays open late into the night and there are films, tours and talks at most locations. I would definitely go again!!
General Info: http://www.langenacht.ch/langenacht15/
Program and Venue list: http://langenacht.ch/hub/files/uploads/LN15_Booklet.pdf