"It is widely accepted that in 1971, a group of teenagers at San Rafael High School in San Rafael, California, calling themselves "The Waldos", used to meet every day after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue. The term became part of their group's salute, "420 Louis," and it eventually caught on more widely. Many cannabis users continue to observe 4:20 as a time to smoke communally. By extension April 20 ("4/20" in U.S. dating shorthand) has evolved into a counterculture holiday, where people gather to celebrate and consume cannabis." 

Who didn't know about this holiday? I understand if people out of the U.S. don't know it but anyone that lives here should know it.:P