Bevrijdingsdag (Liberation Day) is celebrated every year on May 5. It marks the end of World War 2 for the Netherlands.
There are usually several (open air) concerts in the country, and usually also in Amsterdam.
Another tradition is the floating open air concert in the river Amstel, in front of theater Carré. This is a free concert, given in the evening, and attended by the King and Queen. The Amstel will fill with boats with specators, and so will the streets and bridges around. Try to be early for a good view!