I was searching for some information about a Chinese reference, and in passing found a description of Buenos Aires' Night of Museums which took place on Saturday. It looks like a great event, and seems to be based on a similar idea from Germany. One of the reasons I've become a more frequent visitor to the British Museum this year, is the fact they're open on Thursday and Friday evenings. Not sure if I'd have the stamina for a very late night opening, but I like the concept.
What I did find surprising is that I can't find any obvious reference to this event on British or American sites, although I've only done a quick search.