420 was being celebrated before Jerry passed.
There's a mystery surrounding 420.
The most prevalent rumor is it started out in Cali. A bunch of community college stoner would gather everyday after/between classes at 4:20 and enjoy some smoke. Slowly the gathering grew & slowly spread to other campuses. It sounds legit to me, as growing up on the east coast, I had never heard of it. Wasn't until about '00 or '01, I heard about it. It's still not well known out on the east coast, which leads me to believe it started somewhere out west.

Just looked it up on Wikipedia: 420 (cannabis culture) - Wikipedia, the free encyclopedia

Pretty much similar to what I stated above, except they say it was high school kids.