Amsterdam Queen's Day

If you want to join the festivities, yes. If you want to get around easily and see the usual tourist highlights, no.

To clarify, Queen's day is an all day & night street party. All the streets in the city center will be closed to car and tram traffic. All of the tourist sites (like museums) will also be closed for the holiday.

We were there in April 2010. It begins earlier in the week with people marking off "their spot" on the sidewalks. This is the day when you can have you own private flea market and the government looks the other way. Many people sell homemade goodies, some kids dance or act for money, some people sell hats, CDs, books etc. We lived in an apartment where two canals met so we could see all the action. The loud music starts the night before, but I think it stops at 10 or 11. The next day the town is packed. Try to wear something orange. The Dutch go around and around the canals in their boats with booming loud speakers.
Then in the evening everything just stops at a designated time. Great cultural experience. You can see it on youtube. Of course, there are shops where you can buy something orange. Have fun!