April 20th is the International holiday for the celebration of Marijuana. The origin of this celebration is said to come from a group of California high school students who gathered at 4:20 to smoke pot. One of the teens went on to become a Grateful Dead roadie which eventually lead to the discovery of the practice by the editor of High Times magazine. And the rest is history. Here is a link to the Wikipedia article.
Why do we care? Because Mary, or more specifically Mary Jane, is another of so many slang terms for Marijuana. Why so many? We like this article from Time.com, Why so many names for weed.