Yes iamsterdam.com should have all the info as the date nears.
The opera performance is usually advertized quite short notice on their website http://www.bredewegfestival.nl/ (in Dutch). It's a neighborhood festival that grew and grew and is now a 'place to be' for many Amsterdammers who want to escape the non-Amsterdammer crowds in the center. It normally also continues on Queensday and I expect it to be the same for King's Day (Kingsday?).
Some of the major areas:
- Jordaan, canal belt; and Apollolaan and surroundings (south) for the flea markets, the dedicated shoppers start at 8am to find the best bargains. But basically the entire city has people selling old junk on the street.
- Jordaan and canal belt for street parties, it can get very crowded in the narrow streets (if you want to go to another part of town, follow one of the designated walking routes which use wider streets)
- There are some huge mainstream parties on Leidseplein and Museumplein, I'd avoid those... these mostly attract the young, rowdy youths from outside Amsterdam. I'd also avoid Dam Square which is just very crowded.
- Vondelpark is for kids: children perform music, dance or sell stuff, or do games to earn money (for example shooting candy high in the air and you can win money if you catch it in your mouth... if you half miss, your face will be splattered with white cream from the candy... things like that)
- Bredeweg / Hogeweg for flea markets, great food sold from stands by restaurant owners, live performances, mellow vibe.
Even though most people seem to think it's normal to throw trash on the street, please don't... there should be plenty of extra garbage bins and containers you can use. Us Amsterdammers pay for the cleaning up afterwards...
If you're up for a party and some local fun, I definitely think that weekend would be better than Easter weekend.
Do not try to use the train into or out of Amsterdam on that day.
Edited: 11:37 am, September 21, 2013