It is widely accepted that in 1971, a group of teenagers at San Rafael High School in San Rafael, California, calling themselves "The Waldos", used to meet every day after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue.   The term became part of their group's salute, "420 Louis," and it eventually caught on more widely. Many cannabis users continue to observe 4:20 as a time to smoke communally. By extension April 20 ("4/20" in U.S. dating shorthand) has evolved into a counterculture holiday, where people gather to celebrate and consume cannabis. 
There are also many apocryphal urban legends attempting to explain the origin of the term. Two of the most common of these are that 420 refers to the number of active ingredients in cannabis, or that it is police dispatch code for cannabis. In actual fact there are around 315 active chemicals in cannabis, varying depending on the exact plant used, and 420 has never been verified as the police dispatch code for anything in any locale.
The widespread popularity of 4/20 celebrations in the U.S. has brought about calls advocating for the reform of American marijuana laws. The 420 Campaign urges individuals to become involved in the political process and the drug policy reform movement. Specifically, the Campaign calls for leveraging "April 20th as a focal point every year to concentrate pressure on Congress to legalize marijuana." In addition to contacting state and national legislators, people can take action by supporting organizations—such as NORML and MPP—that represent the interests of marijuana users and other concerned citizens.