On October 21st:

 1805   Battle of Trafalgar

One of the most decisive naval battles in history, the Battle of Trafalgar, took place on this day. British Admiral Horatio Nelson led the Royal Navy to a spectacular victory over Napoleon’s combined French and Spanish fleets off the coast of Spain. Nelson’s strategic brilliance and courage in this battle made a lasting impact on naval warfare.

 1959    Guggenheim opens in NYC

This contemporary art museum designed by Frank Lloyd Wright formally opened in Manhattan on this day.

 1967    The March on the Pentagon

On this day there was a massive demonstration against the Vietnam War where over 35,000 people marched from Lincoln Memorial to the Pentagon to protest US involvement in Vietnam.