From Wikipedia:

Although many diverse theories exist to explain the origin of the term, it is widely accepted that in 1971, a group of teenagers at San Rafael High School in San Rafael, California used to meet after school at 4:20 p.m. to smoke cannabis at the Louis Pasteur statue.[3][4] The term became part of their group's salute, "420 Louis!",[5] and became popularized in the late 1980s by fans of The Grateful Dead.[6] 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.[7][8][9] In some cities, this celebration coincides with the celebrations of Earth Week.

Don't know how valid that is, but whatever.

I used to get text msgs from all my pot buddies at 4:20 am and pm wishing me happy 420. Gets kinda old. Big parties on 4/20, starting at 4:20 and going until past 4:20 or whenever everyone's so stoned they pass out. In my time, I suppose I enjoyed the novelty of the idea of having a stolen 420 sign... not actually stealing one... ((I'm too innocent for that sort of thing ))

Hmmmmmmmm I wonder if there will be fewer pot related traffic incidents on Highway 440 than there were on 420?

Would be a good place to sit and smoke a fatty...