Until 1961, the Kingdom of the Netherlands recognized May 4th as a day to honor those who died for their country during World War II and titled it as “Remembrance of the Dead” day. After 1961 it was decided that all those who had died since the the outbreak of the war should be honored as well. Now no matter where the member of the armed forces may have died, if it was a war or peace keeping mission is remembered at 8:00 p.m. on the 4th of May.
Throughout the country as well as in other parts of the world, such as Saint Martin, people will gather at monuments or watch on tv the ceremonies. The largest of which is at the National Monument on the Dam, where 15,000 people will gather and over 3 million people will watch via television a program starting at 7:50 pm this year with the traditional 2 minutes of silence at 8:00 pm. During the day flags will have flown at half staff until after this time and will then be raised to the full height of the flag pole to the music of the Dutch national anthem. At the Dam a parade will begin at 8:18.
I was unable to find why this day and time have been chosen, the only clue I have is that the liberation of the Netherlands is the 5th of May which would make sense to me but I can not be sure if this is the reason or not.
Officile site (translated from Google)