April 16, 2007: VT Shooting Spree. 33 were killed (including shooter)
April 18, 1983: Suicide bomber destroys Lebanon embassy. 63 deaths
April 19, 1989: Turret explosed on USS Iowa. Kills 47.
April 19, 1993: The Branch Davidian Siege ends in the deaths of 81 people
April 19, 1995: Oklahoma City Bombing kills 168
April 20, 1999: Columbine Massacre. 15 were killed (including shooters)

Don’t forget April 15, 1912: Titanic sinks. About 1,500 die.