420 or 4:20 or maybe 4/20 is a secret code in cannabis culture which denotes to the consumption of cannabis or marijuana, especially smoking it around 4:20 pm and also celebrating cannabis (marijuana) on the date April 20. Cannabis, also known as marijuana is a psychoactive drug derived from the Cannabis plant basically designed for medical or recreational use. It can be consumed by smoking, vaporizing within the food or as an extract. Its geographic origin is Central and South Asia.

Now you must be thinking how this weird term must be originated and from where? So here is the answer to your question-

In 1971, 5 San Rafael high school students named as Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich who called themselves as “Waldo’s” used this secret code in order to connect with each other for completing a single objective i.e. searching for abandoned cannabis crop about which they had learned based on the treasure map which was made by the grower. Later on, it became their chosen hang-out spot. These Waldo’s had labelled the Louis Pasteur statue on the grounds of their high school which they chose as their meeting place and decided 4:20 pm as their meeting time. They used to refer this plan with the motto "4:20 Louis". Later on, they dropped the term Louis and used 420 as a code word for marijuana-related stuff. Now one more doubt had occurred in your mind that how this code word got a worldwide recognition? Then here is the answer-

According to Mike Edison, Steven Hager of High Times was responsible for taking the story about the Waldos to "mind-boggling, cult-like extremes" and "suppressing" all other stories about the origin of the term. Hager wrote "Stoner Smart or Stoner Stupid?" in which he labelled 4:20 p.m. at the socially accepted hour of the day to consume cannabis or marijuana. He featured the early spread of the phrase to Grateful Dead followers, who were also linked to the city of San Rafael.

From that point of time, 20th April has been marked as an International counterculture holiday, where people cluster to celebrate and consume cannabis. Founder of Seattle's Hempfest, Vivian McPeak states that 4/20 is "half celebration and half call to action" whereas, Paul Birch calls it a global movement and suggests that one can't stop events like these. On this day thousands of people illegally consume cannabis at several locations to celebrate 420 and end marijuana prohibition. 

Observances of this day had been made in several parts of the world.  For example

IN NORTH AMERICA

• Hippie Hill 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 Park
  
  
• Montreal, Quebec, at Mount Royal monument
  
  
• Edmonton, Alberta, at the Alberta Legislature Building
  
  
• Vancouver, British Columbia, at the Vancouver Art Gallery

ELSEWHERE

• London, The United Kingdom in Hyde Park
  
  
• University of Colorado Boulder
  
  
• University of Otago, Dunedin, New Zealand

It is believed that in future at some point in time, High Times eventually picked up the term and published it throughout their magazine and that’s how this term is known globally. Today we use 420 less as a secret code and more as a celebratory roar. Everyone knows that 420 denotes something related to marijuana and I doubt that it will ever change.