What's the deal with 420?

What is the origin of the pothead New Year, AApril 20th? I’ve heard various claims. Some lack any form of evidence, even from reliable sources, such as the claim that it is the police code for consumption of marijuana. Others are obscure and derogated, an example being that it is the toxicity level the human body can handle (LD50) [understadibly hard to disprove]. Even more, though, are overexagerated, like the statement that the plant must reacha temperature of 420 degrees F to burn [ummm… no. Bics don’t go that high]. Then there are the seemingly silly: ag roup called the Waldos would meet at a statue of Louis Pastuer after school activities to go smoke up [Funny that I accept this one the most]. Could you please help lay this confusion to rest? Thanx.

My stoner friends tell me that 420 is the police code for possession of marijuana. Yeah, my stoner friends…

Actually, this site says no:
http://www.phish.net/faq/n420.html

This was done to death at http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=105420:p

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Did someone say 420?