The term 420 is used widely in Cannabis culture as a time to get high, a national day to get high, and cannabis itself.

There are a lot of different theories about how the word was first invented:

:: San Rafael theroy ::
In the early 1970's, a group of teenagers who attended San Rafael High School in San Rafael, California used to get high every day at 4:20 in the afternoon under the Louis Pasteur statue.

:: Bob Dylan theroy ::
In Bob Dylan's song, 'Rainy Day Women #12 && 35,' the lyrics contain 'everybody must get stoned.' 420 comes from the title, as 12 * 35 = 420.

:: Highway 420 ::
In Ontario, Canada in the 60's, marijuana grew freely on one of the Highways. This highway was later renamed in 1972 as Highway 420.

:: The Date 4/20 ::
April 20th is coincidentally (or not..) the birthday of Adolf Hitler who was born on April 20, 1889. It is also interesting to note that the Columbine High School shootings also occured on this date in 1999.

There are many other theories that have been debunked such as the Bob Marley theroy and the Police Code theroy.