Here is a little info on the 'tradition'
http://www.snopes.com/language/stories/420.htm
Its an urban legend kind of thing. It is also why the kids choose to do the school shooting on that day. Columbine?
I've heard a variety of stories about the origin of the usage of "420" as a euphemism for smoking pot, ranging from that being the number of chemicals in the plant (also not true) to it being the time at which some high schoolers in California or somewhere met after school to get high (which may indeed be the real source of the code).