With April 20th directly in the rear view, and the parties and protests that accompanied it, you may be wondering how “four twenty” came to be associated with imbibing marijuana.
This infographic relates the somewhat underwhelming story of a group of high school stoners that would meet after school at 4:20PM to smoke weed. This story, while not fantastically well corroborated, does seem to have better evidence on its side than the other various origin myths. In addition to the myths listed on the infographic, I've heard that April 20th is Bob Marley's birthday, which is actually February 6th.
Accepting the infographic's (and Wikipedia's) story as truth-y enough, it seems very fitting that the Grateful Dead would be involved.
April 20th, 2016 saw people around the world smoking marijuana publicly in protest, and in some cases, in celebration. In the US, especially, marijuana legalization seems to be picking up steam. This year, 2016, Pennsylvania became the latest state to legalize weed.
If you are interested in a deeper look at legal specificites of marijuana use in the United States, check out this Scheduling of Marijuana infographic.