no one is sure why but this is the wikie http://en.wikipedia.org/wiki/4/20 it used to be better but wikipedia took allot of stuff of the artical for some reason
@CrazedPotHead: it's different in other parts of the world, like here in sweden we write the date day/month which is then 20/4 and not another of these threads, feels like i'm reading these kind of thinks all day long sometimes, and NO ONE KNOWS were it originated..