quote from hightime:
420 TIMELINE
420 is not so much a time or place as it is a state of mind. A stoned state of mind, to be specific. In the past, HIGH TIMES has reported on various competing theories surrounding the appearance of this unique bit of smoker slang, but as the following timeline clearly shows, the controversy has been resolved. The original story of 420 begins with five fellow students - all of whom assumed the nom de pot Waldo - who met after school at the appointed hour to smoke some of Californias finest, and occasionally searched for a hidden pot feild they'd heard about in the hills outside town.