OK, here it is. Multiple theories...
'What does 420 mean? There are varying theories on the origin of 420. Some say that 420 originated from a police code that announces marijuana use is taking place.
Yet another story is that a group of guys (Waldo's) in the 1970's made 4:20 their official meeting time to smoke marijuana after school. Whether or not 4:20 p.m. is the best time of day for your first hit depends on your own body, your own needs. Some folks feel that waiting until 4:20 enhances ones appreciation of the herb. Of course, your mileage may vary.
In the 21st Century, 420 is firmly established as a code amongst tokers, a time of day and even sort of a toker's New Year's Day. It's in our culture now and only time will tell where it ends up.
April 20th (4/20) is another usage, meaning that it is time for to plant before the summer.'
http://www.concept420.com/what-is-420.htm