IIRC 420 is a US police code for 'weed smoking people smoking weed'
Today is (according to the yanks, who are wrong) 4-20-2013 (it's actually 20-4-2013)
Hence 20th April is 4-20 or 420.
This leads to a day where something about marijuana. Probably.
Obviously over here, weed is illegal, so it just becomes (yet another) reason to go out and celebrate, just like the great reasons most brits use to go and get hammered, such as international pirate day, It's my cat's Birthday, I didn't burn the toast thismorning so I'm going out celebrating. etc.
Something about not doing drugs and staying in school, kids!