Allow me to quote from Wikipedia, it's easier for me to explain it in English (besides: some info I didn't even know myself!):
---------
Remembrance of the Dead (Dutch: Dodenherdenking) is held annually on May 4 in the Netherlands. It commemorates all civilians and members of the armed forces of the Kingdom of the Netherlands who have died in wars or peacekeeping missions since the outbreak of World War II.
Until 1961, the commemoration only related to the Dutch victims of World War II. Since 1961, the victims of other military conflicts (such as the Indonesian National Revolution in Indonesia) and peacekeeping missions (such as in Lebanon or Bosnia) are remembered on May 4 as well.
Traditionally, the main ceremonies are observed in Amsterdam at the national monument on Dam Square. This ceremony is usually attended by members of the cabinet and the royal family, military leaders, representatives of the resistance movement and other social groups.
At 8:00 p.m., two minutes of silence are observed throughout the Netherlands. Public transport is stopped, as well as all other traffic. Radio and TV only broadcast the ceremonies from 19.00 until 20.30. Since May 4, 1994, the flags, having hung at half-staff during the day, are then hoisted to the sounds of the Wilhelmus, the Dutch national anthem. Since 2001 the new protocol says it is correct to let the flag hang half-staff.
The main celebrations in Amsterdam are broadcast by the public broadcasting company NOS, but there are ceremonies in other cities and places as well. Especially notable are those at the Waalsdorpervlakte near the Hague where many Dutch resistance fighters were executed during the war and at the war cemetery Grebbeberg, which are broadcast by the commercial broadcasting companies. In many towns, before or after the two minutes of silence, people gather around a monument, listen to speeches, and lay down flowers to remember the dead.
One day later, on May 5, Dutch people celebrate the liberation from the German occupation between 1940 and 1945.
