there is no actual reason, but the most plausable, reasonable and most mentioned reason would be that there were a bunch of friends in college trying to figure out when they could all get together to smoke (as usual they didn't all have the same classes, times ect ect.) thus they eventually came up with 4:20, after awhile it spread....

People do know how it started. It was started by a group of kids in the 70's who called themselves "the Waldos". They would get high outside of their school, San Rafael High School, near the statue of Louis Pasteur, and would say "420 Louis" as their secret code to one another.
you can't prove it's here-say nor can you prove those fliers weren't made (or even made up) just so they could be 'the creators of 420'...
slipknotpsycho Reviewed by slipknotpsycho on . Origins of 4:20 So... I'm sitting here, noticing what time is rapidly approaching, and I pause a second to wonder about how this tradition started. Anyone? Rating: 5