April 20 is observed by many Americans as the unofficial holiday of weed-smoking. According to marijuana news site High Times, the story of the day began in the 1970s, when a group of high school students began meeting up at 4:20 in the afternoon to smoke wacky tobacky outside of their high school — though rumors also suggest that "420" was the police code for "smoking in progress," as LA Weekly debunked.
The lore of 420 was passed "stoner to stoner," according to High Times, until it became pretty much an actual event celebrated on college campuses and in city parks all around the country — but is 420 international? It seems that 420 has actually spread around the globe, like a very mellow epidemic.
On April 19, 2015 (a Sunday) the Independent reported that "a cloud of cannabis smoke ... billowed up from the 1,000 people gathered at Hyde Park in London at 4:20 p.m." So 420 has made it to the U.K., even if they do sometimes celebrate a day early.
According to CannabisCulture.com, in 2014, 420 marijuana rallies were held all over the globe, including in Peru, Austria, New Zealand, Canada, Iceland, Denmark, Italy, South Africa and Australia. So if past years are any indication, this year 420 will continue to be a worldwide celebration.