TIL that the Boston Marathon (2013), Virginia Tech Shootings (2007), Columbine Shootings (1999), Oklahoma City bombing (1995), and the Branch Davidian siege (1993) have all occurred in the third week of April.

http://www.wtop.com/1326/3292534/Third-week-of-April-Oklahoma-City-Columbine-Boston-Marathon-Bombings