It is the Dutch Queen's sunny-weather birthday at the end of April. It is a day of enormous joy in the Netherlands as it is a public holiday and the Dutch citizens, adults and children alike, are allowed to sell their junk on the streets without having to pay taxes on the profits. Most people wear orange, a lot of faces are painted with the Dutch flag, and naturally a huge volume of Heineken is consumed.
The Dutch Consulate and Summit Hotel will be hosting a fun and refined Queen's Day on 30th April and the invitation is attached.
http://en.wikipedia.org/wiki/Koninginnedag