I have always heard, at least around Michigan, that 420 referred to April 20th as a day to go out into the woods and randomly plant marijuana seeds for a fall harvest.
I'm sure it is. Here's some additional data on the origins of the significance of "420":
"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.
Whatever the real story is, 420 has been an important part of the marijuana culture since the 1970's. The significance of 420 has been kept underground and is mostly known only among marijuana smokers. Many non-smokers aren't aware of the symbolism when they see someone wearing a T-shirt or baseball cap that says 420 across the front.
When the 420 icon is somehow discreetly worked into a mainstream product like a film, marijuana users take notice. The film Pulp Fiction is rumored to have had all clocks throughout the movie set to 4:20. Marijuana smokers familiar with the symbol picked up on it—most people, however, did not.
While some marijuana smokers are using 420 as a code that enables them to openly speak about marijuana in front of parents or teachers. 420 has been to some, a sacred symbol for nearly 30 years.
Simply put, 420 is a symbol of cannabis and its culture. Today, April 20th events are international, and 4:20 pm has become sort of a world wide "burn time"."
Earth day? The day of the Gulf oil spill?
Who knew? It was all about reefer.