Koninginnedag, or Queen's Day, was begun to celebrate queen Juliana's birthday and current queen Beatrix decided to keep the same date - April 30. Its bedlam. The entire city becomes one gigantic flea market and street party, where anyone can set up a stall and sell all that junk they've been hoarding in the attic.
And they do. The city centre gets totally jammed up with people wearing orange wigs and feather boas or whatever the orange hit of the year is. (Orange being the official Dutch colour, as in House of Orange and the royal family). The Dam, Rozengracht and round the Leidseplein tend to be set aside for commercial stall holders, so are not so interesting. Everyone with a boat tends to take to the water as well.
Don't be afraid to haggle over prices. Mind you, if you are a real bargain hunter rather than a party animal you should probably leave town and head for some village full of rich people.
The Vondelpark is a bit calmer and set aside for children, so be prepared to listen to tortuous violin playing and put a few cents in the politely proffered tin.
I've sold stuff on Queens Day and bought stuff on Queens Day. We still have a very large 1930s coat rack bought four years ago and still waiting to be mended.