Alternet explains it all for you!
Go to the link to find out how. And on the way, "Smoke Two Joints..."
Happy 4/20! The True Story Behind Stoners' Favorite Number
Warren Haynes, the Allman Brothers Band guitarist, routinely plays with the surviving members of the Grateful Dead, now touring as The Dead. He's just finished a Dead show in Washington, D.C. and gets a pop quiz from the Huffington Post.
Where does 420 come from?
He pauses and thinks, hands on his side. "I don't know the real origin. I know myths and rumors," he says. "I'm really confused about the first time I heard it. It was like a police code for smoking in progress or something. What's the real story?"
Depending on who you ask, or their state of inebriation, there are as many varieties of answers as strains of medical bud in California. It's the number of active chemicals in marijuana. It's teatime in Holland. It has something to do with Hitler's birthday. It's those numbers in that Bob Dylan song multiplied.
The origin of the term 420, celebrated around the world by pot smokers every April 20th, has long been obscured by the clouded memories of the folks who made it a phenomenon.
The Huffington Post chased the term back to its roots and was able to find it in a lost patch of cannabis in a Point Reyes, California forest. Just as interesting as its origin, it turns out, is how it spread.
It starts with the Dead.
Incidentally, if you drop the "zero" at the end, the remaining numbers are the answer to the question, from HHGTTG: "What is the meaning of life?" Which, of course, is the kind of thing a 'stoner' would notice...