Whence "420"

Today is April 20, the day pot-smokers celebrate the plant in North America and New Zealand. This morning playing Bananagrams at the local coffee shop one of the players dropped the term "420" in conversation about high-schoolers and their coded misdemeanors.  Being a totally clue-less person, I asked what the term referred to and she was either not sure or reluctant to show too much knowledge.

This afternoon reading the Andrew Sullivan blog, I came across the supposedly authoritative answer to the source of the term.  A link took me to the following video on the Huffington Post San Francisco blog site:

So, with the same determination that the Oxford English Dictionary brings to finding the first recorded use of a word, the Huffington Post tracked down the first use of "420."

Here is a picture from Golden Gate Park in San Francisco on April 20, 2010: