4:20 or 4/20 the time or date to smoke weed.
The earliest use of the term began among a group of teenagers in San Rafael, California in 1971. Calling themselves the Waldos, because "their chosen hang-out spot was a wall outside the school," the group first used the term in connection to a fall 1971 plan to search for an abandoned cannabis crop that they had learned about. The Waldos designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 pm as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis". Multiple failed attempts to find the crop eventually shortened their phrase to simply "4:20", which ultimately evolved into a codeword that the teens used to mean pot-smoking in general.
High Times editor Steven Hager wrote "Are You Stoner Smart or Stoner Stupid?" in which he called for 4:20 pm to be the socially accepted hour of the day to consume cannabis. He attributes the early spread of the phrase to Grateful Dead followers, who were also linked to the city of San Rafael.
April 20 observances
April 20 has evolved into a counterculture holiday in North America, where people gather to celebrate and consume cannabis. Some events have a political nature to them, advocating for the decriminalization of non-medical cannabis in the United States
Observances have been held in San Francisco's Golden Gate Park near the Haight-Ashbury district, the University of Colorado's Boulder campus, Ottawa, Ontario, at Parliament Hill and Major's Hill, Montréal, Québec at Mount Royal monument, Vancouver, British Columbia at the Vancouver Art Gallery, Auckland, New Zealand at the Daktory and Dunedin, New Zealand, at University of Otago