I just found out from my 13 year old daughter that 420 is quite a significant number based on the pot culture and commonly known by teenagers and pot smokers around the world for years. Google confirms this.
I’m really surprised that I could have been so totally unaware of this popular term.
Was I all alone in this particular ignorance?
Not at all. I hadn’t the faintest when I saw 420 threads on another message board.
I still don’t remember, I’m off to google.
Yep. Just you. 
Kidding, I believe it’s the police code for it, but that might have been becuase of something I read recently that may in fact have said the opposite.
Yep, I in fact got it wrong relavent snopes link. For any one too lasy to follow the link, a group of students down the road in San Rafael, CA started to use it as slang for when they’d meet to light up.
Heh, the whole 420 lexicon is only followed by poser Stoners and the hardcore Potheads (the ones that act stoned even when they’re not). I think it gets more brouhaha than it deserves.
Besides, it’s more fun to think of one’s own terms… wanna go dreeze the shoofooballoo?
Ah, I had never heard of it either, until. some time ago, there was at least one thread on it on SDMB. Of course, the Search function is not pleased at being asked to search for “420”, it seems.
I had some vague notion that the explanation was to do with a film or a police code, but I simply don’t know.
Here is a link.
And remember Dopers, 420/pot/marijuana/wacky tobaccy/weed/hash/grass/puff/green is illegal in the United States.
I can say, anyway, that I never heard it in Britain.
Don’t feel so dense.
I was at my hundred-and-umpteenth Grateful Dead concert when I had to ask a teenager what “4:20” on his baseball cap meant.
His reply: “Time to smoke pot, dude!” :wally
It’s not easy to be made to feel square and unhip while you’re standing in a parking lot before a Grateful Dead concert, partaking of the various and sundry chemical alterants therein, legal and not quite so. But he did it.
I grew up in the '70s and was a musician to boot. No way to avoid coming into contact with people who smoked pot. I’d never heard anyone mention ‘420’ until just now. Maybe it was a regional thing?
Man! What a dissapointment, I thought this was all about how you were going to go on a diet.
I think it 's a short version of “The whole 420 yards.”
My pothead son, currently serving time for said proclivities, uses 420 in all sorts of ways…email addresses being the most common. I see it everywhere these days…and the kids think they are being so cute and clever that we won’t know what they mean! In fact, I had a guy have me engrave a clock for his girlfriend that read, “we’ll always have 4:20” !! (worst thing was, he was a guy who had assaulted my daughter and gone to jail because of it…he didn’t recognize me, and I recognized him only after I had taken his order and engraved it. I was shaking so hard when he picked it up, hoping he wouldn’t realize who I was.)
In Ontario, Highway 420 is the short highway that joins the Queen Elizabeth Way to the Rainbow Bridge in the city of Niagara Falls. 
As someone who has never smoked pot, never been offered pot and wouldn’t have the slightest idea where to get pot, and yet I still know what 4:20 means, I can’t believe you didn’t know it either!
It was a gag on the Simpsons and everything, jeez!
Yes. I’ve never even smoked a regular cigarette let alone pot, but I know the significance of 420. Nice rock you’ve got, though 
The first I remember hearing it was after the Columbine shootings, which took place on April 20. There was a lot of talk then about its connection to marijuana and also that it’s Hitler’s birthday.
I got my nose pierced at a shop called ‘420’. They gave us a sticker saying ‘I got pierced at 420’. It was green, and shaped like a cannibis leaf.
That’s really all I have to contribute, other than to say that even as a pre-teen I knew what it meant, and I grew up in rural Ohio.
Carry on…
I smoked long ago and not much, but my bf back then was the connection. I had heard it. I also saw a bumper sticker recently that said
“What Would Jesus Do At 4:20?”
The local rock station often announces:
“It’s 4:20…time to get Stoned!”
when they play the daily Block of the Rolling Stones.
At the University of Vermont, there is (was, anyway) a big unofficial gathering on April 20th at 4:20pm outside the library, of all places. My senior year they even had a band play, and people usually give out pamphlets promoting legalization. Everybody would then light up at 4:20 (outside, on the grass), thinking that the small Burlington police force couldn’t arrest all of them. They were right, as far as I could tell.
I’m pretty sure I hadf heard the expression while in High School, as well.