Since Willem Alexander acceded to the throne on April 30 2013, Holland no longer celebrates Queen's Day, but King's Day.
Under his mother's reign (Queen Beatrix), there was Queen's Day, celebrated on April 30 (which was Queen Juliana's birthday, the mother of Beatrix).

April 27 is King Willem Alexander's own birthday, so that day has been chosen as the future King's Days. An exception is made if April 27 is on a Sunday, in which case King's Day will be held on Saturday April 26 (e.g. the first King's Day, in 2014).

What will not change is the large party that takes place in Amsterdam every day on this day. The basis of the party is a huge garage sale: on King's Day everyone can sell whatever they like on the streets, and this is done throughout the Netherlands, including Amsterdam.

In Amsterdam, in addition, King's Day is also a huge street party, which attracts many tourists from all over the world. The number of visitors ranges from approximately half a million and a million - in certain parts of town it can get pretty busy (particularly the old center and the Jordaan area).
King's Day is definitely one of the largest yearly events in Amsterdam.

It gets busy in the center of Amsterdam from about noon, until about midnight.

Since the Royal House in the Netherlands bears the name 'Huis van Oranje' (House of Orange), the colour orange is seen everywhere. Orange clothes, wigs, glasses, hats, etc. are all over the place.