It took me a bit to track down but this is what I could find:

To North americans, the phrease 4-20 which is pronounced fourt-twenty, means the smoking/eating of marijuana at a particular time. Where the phrase came from is unknown but today when someone says four-twenty it is general knowledge of what comes next. Smokers will get their tools ready on April 20th every year and celebrate on a global scale.

The origin of the word 4:20 as I said cannot be tracked down to a precise date however from what I gater it first became widely used in the 1970's. A group of high school students that went to "San rafael HS", located in california met eachother everyday after school at 4:20pm. To smoke pot at
the Louis Pasteur statue.

Let me know if you find anything else.