While probably every day marks several historic occassions I am always struck by how certain times of year seem to mark a confluence of events.
Consider the middle part of April
April 12th marks the start of the Civil war, the death of FDR and the first man in space
April 14th-15th marks both the Titanic and the Lincoln assassination
April 18th marks the 1906 San Francisco earthquake
Mid April also marks Apollo 13 mission, the Oklahoma city bombing and the battles of Lexington and Concord.
If you extend it to the first half or so of April (April 1-19) you get the end of the Civil war, the death of President William Henry Harrison, the assassination of MLK, US entry into World War 1 and US entry into the Spanish American war.
Pretty interesting eh ?