The real story behind the origin of why the number 420 is synonymous with weed is actually pretty simple and well-documented.
For a journalism-styled breakdown of the long, not-sordid-at-all tale about the herbal significance of the three digits, it’s hard to find a better version that the one The Huffington Post ran a few years back.
But for the TLDR–too long, didn’t read–crowd, it’s easy enough to still down the essentials into a few sentences.
Basically, back in the ’70s, a group of high school students–who called themselves the Waldos–in San Rafael, California used to get together after school to smoke. 4 :20 was the customary time to meet up, so they started using 420 as slang for weed. After some of the kids became friends with members of the Grateful Dead, they brought them in on the lingo, and Deadheads spread 420 round the world. It’s not anything more complex than that.
Check out the video to see the Waldos, or at least dudes who claim to be the Waldos, tell their story. And watch the Funny or Die version for a less accurate, but far more funny explanation.