April 19th is Dutch-American Friendship Day.
Dutch-American Friendship Day remembers the day in 1782 that John Adams, the second president of the United States, was received by the States General in The Hague and recognized as Minister Plenipotentiary of the United States of America.
It is also the day that the house John Adams purchased at Fluwelen Burgwal 18 in The Hague became the first American Embassy in the world.
In 1982, two hundred years later, President Reagan proclaimed April 19, 1982, to be Dutch-American Friendship Day (read the text here: http://www.presidency.ucsb.edu/ws/index.php?pid=42385).
Fifteen years later, in 2007, Congress made this a repeating commemoration, and on March 12, 2007 the House of Representative officially established Dutch-American Friendship Day:
"The U.S./Dutch relationship has stood the test of time and has strengthened in the crucible of conflict as the Dutch have stood beside us in times of peace and war. The Dutch supported us in our war for independence. Sixty years ago Dutch and American servicemen stood side by side during World War II and today the Dutch stand by us still in the Global War on Terror.
The debt we owe to our Dutch friends is seen not only in our people, and in the persons of such famous Dutch Americans as Presidents Martin VanBuren, and Theodore and Franklin Roosevelt, but also in our experience as a Nation. Our traditions of religious freedom and tolerance as well as our system of government, all have spiritual and legal roots in our relationship with the Dutch Republic. "
(Read more here: http://thomas.loc.gov/cgi-bin/query/R?r110:FLD001:H02415)
Dutch-American Friendship Day is less well known than its cousin, Dutch-American Heritage Day (in November). Also, in April many Dutch clubs and organization in the States are focused on organizing their Queen's Day celebration (April 30th).