Here's what Wikipedia has to say about it. There was the 1985 ATF Siege and Columbine. It was Hitler's birthday (and, for what it's worth, it's apparently Crispin Glover's birthday as well. And then of course there's the whole weed thing. Apparently The Oklahomka City Bombing and Waco both happened on April 19th. Dylan Klebold and Eric Harris mentioned both Oklahomka City and Waco as motivations for Columbine.