420. Never heard of it but I never was much of a pothead. I had to google it. The Huff Po has a good article on the origins of it....basically started as code among some California high schoolers that they were going to meet at 4:20 pm to smoke some dope...and it spread from there to where now potheads everywhere celebrate on April 20th of each year. haha...cool. Here is the link to the article.
What 420 Means: The True Story Behind Stoners' Favorite Number