1. Summer Solstice: June 21 marks the summer solstice, the longest day of the year in the northern hemisphere. In New York City, the day is celebrated with various events, including the annual solstice yoga in Times Square.

2. Father's Day: In the United States, Father's Day is celebrated on the third Sunday in June. Since June 21 is the summer solstice, it is uncommon for Father's Day to fall on this date. However, every few years this does occur, such as in 2020.

3. World War II: On June 21, 1945, the final battle of World War II in Europe was fought in New York City. The battle took place at the Brooklyn Navy Yard, where the crew of the German submarine U-853 surrendered to the United States Navy. This was the last engagement between submarines of the Allied and Axis powers in the Atlantic theater.

4. Stonewall Inn Riots: On June 21, 1969, the Stonewall Inn riots began in New York City. The riots sparked the modern LGBTQIA+ rights movement and led to the formation of various organizations such as the Gay Liberation Front and the Gay Activists Alliance.

5. Manhattanhenge: Twice a year, the sunset aligns perfectly with the east-west streets of Manhattan, creating a phenomenon known as Manhattanhenge. This occurs around June 21 and July 12 each year, and is a popular event for photographers and tourists.