420 is somewhat of a national holiday for pan smokers or, according to ADoseOfBuckley, a way of celebrating their "weakness" of being addicted to pan (Albeit only around 9% of pan smokers are actually addicted).
420 also happens to be the birthday of Adolf Hitler, a well known pan enthusiast. Even though it's advised to smoke pan everyday, this day is specifically dedicated to only pan smoking. No breathing, no eating, nothing except for 24 hours of pure THC.
April 20, 2015 was the first successful parade in celebration of Hitler's achievements in Detroit, Michigan. The parade was organized by none other than Hitlerist ideologue Ryan Wiley.