May 4 is traditionally the National Remembrance Day in the Netherlands.

The day used to be a rembrance day for the victims of World War II only, but in recent years, this has officially expanded to all Dutch victims of all wars since the outbreak of World War II, in the Kingdom of the Netherlands, or anywhere else.
Although the scope of the rembrance has officially altered, the origin of the day lies in the German occupation of the Netherlands from May 10, 1940 to May 5, 1945.
During these 5 years, of the approximately 140,000 Dutch Jews, somewhere between 104,000 and 110,000 were killed. Many of the victims lived in Amsterdam, the most well-known victim obviously being Anne Frank.

May 4 is commemorated thoughout the Netherlands - all over the country there are gatherings, and marches to war memorials.

The most important one, however, is the remembrance ceremony held every year on Dam Square in Amsterdam.
This starts with a memorial service in the Nieuwe Kerk (New Church), located on Dam Square, attended by the King and Queen. Then the Royal Couple walks to the National Monument on Dam Square, where a 2 minutes' silence is observed from 8pm to 8.02pm.
(Traditionally, public transport in Amsterdam also stops, music in many pubs is turned down and Schiphol Airport arranges that no planes fly over the center of Amsterdam during these 2 minutes).

After the silence, the national hymn is played (the Wilhelmus), followed by a lot of wreath layings (the first one by the King), and several speeches.

On May 4, flags are supposed to be flown half-mast.