Why April 20 became the unofficial weed holiday: the history behind 4/20

For cannabis users, April 20—also known as 4/20—has become a widely recognized day to celebrate marijuana.
But how did a random day in April come to be associated with weed?
What is 4/20?
For cannabis users the number 4/20 refers to April 20, and it has become a globally recognized code to celebrate marijuana.
Yes, it is not officially celebrated as a holiday, but it has its own festivities, deals and special events made to celebrate cannabis culture.
Why is April 20 the unofficial marijuana holiday?
There is not one exact theory about the origins of 4/20 as a marijuana holiday, but there are many widely known.
One of them, states that "420" was the code police officers used in radio to signify marijuana. Another one suggest that Bob Dylan's song, “Rainy Day Women #12 & 35,” where the phrase: "Everybody must get stoned" is repeated. The number 420 would be the product of multiplying 12 by 35.
However, the theory most people believe is the one where a group of high schoolers in California in the 70s was involved. According to PBS, the brother of one of them had been growing weed in the woods near their school, and when he was about to get arrested, he drew a map of the location and gave it to the group.
While the kids never found the marijuana, they reunited precisely at 4:20pm to smoke a joint before looking for the location, and thus the tradition began.
Does Bob Marley's birthday have anything to do with 4/20?
No, it has nothing to do with Bob Marley. Despite what everyone may think, Bob Marley neither was born nor died on April 20. Marley was born on February 6, 1945, and he passed away on May 11, 1981, at the age of 36.
The association with Marley probably began with his links with marijuana, but 4/20 has nothing to do with the reggae musician.