OK, it's Hitler's Birthday. But I doubt that's why potheads celebrate Four-Twenty. In fact, I have no idea what the 4-20 significance is. My best guess is, "Hey dude, it's, ah, 4.20, let's spark a blunt."
Fun weed facts: Most the clocks in Pulp Fiction are set to 4.20 (but not when the kid gets the watch, which must be Tarintino's effort to protect children). The football score on Fast Times At Ridgemont High is 42 to 0. Which must have been Spicoli's idea.
Songs That Celebrate Marijuana Which Aren't the Grateful Dead!