This day is celebrated because the British citizen was given U.S. citizenship on this day in 1963 by President John F. Kennedy.
I know little about him, so I'll give you the best I can about him, then you can see the link all about the man I'll provide at the end of today's blog.
"ON APRIL 9th, 1963, a deeply moved Sir Winston Churchill, sitting in his London home with his wife beside him, watched a satellite relay of a White House ceremony giving him honorary United States citizenship. It had been hoped that he would not only witness the event by TV but would also he able to respond. However, the relay station at Goonhilly, Cornwall, was not ready to transmit and it was decided not to request French help for this special Anglo-American occasion.
In Washington several hundred guests, including Averell Harriman, Dean Acheson and three sons of Franklin Roosevelt, gathered in the White House Rose Garden. A very special guest was 92-year old Bernard Baruch, a close friend of Sir Winston's. Observing from a window and recovering from a stroke was the American President's father, Joseph Kennedy, a former Ambassador to Great Britain and opponent of American involvement in the war. The Churchill family was represented by Randolph Churchill and his son, Winston.
Sir Winston was to share a distinction held only by the Marquis de LaFayette. But it was the first time that Congress had actually resolved that honorary citizenship he bestowed, by the President of the United States, on a foreign national. LaFayette had local citizenry conferred on him by Maryland and Virginia when the colonies became the Union in 1788. Like all other citizens of those states, he then became a citizen of the newly-created United States of America. Churchill, by contrast, had been granted honorary citizenship of eight states: Hawaii, Maryland, Nebraska, New Hampshire, North Carolina, Tennessee, Texas and West Virginia.
President John F. Kennedy praised Sir Winston as a defender of freedom, wartime leader, orator, historian, statesman, and Englishman. The President's opening remarks gave eternity the classic tribute to one of Sir Winston's greatest achievements: "He mobilized the English language and sent it into battle."" (Quote taken from this page: www.winstonchurchill.org
You can find out all about his life at this link: www.winstonchurchill.org