4 / 20 = 0.2
April, 20th, the anniversary of Adolf Hitler’s birthday is also used as a tattoo by racists and neo-Nazis, to affirm their belief in the ideals of National Socialism. A common but very different meaning for "4/20" (or "4:20" or "420") is as a slang term associated with smoking marijuana.
The number 420 is a euphemism for cannabis and its associated culture. The exact origin of the term is unknown, although there is much speculation and many urban legends about its origin. Smoking marijuana at 4:20 or on April 20 has special meaning to some.
Many mistake April 20 as being the day Bob Marley, a famous rastafarian musician, died. The actual date he died is May 11. (yeah, and how wrong, since actually its Hitlers Birthday ;P, see above)
'420' began its sub-rosa linguistic career in 1971 as a bit of slang casually used by a group of high school kids at San Rafael High School in California. '420' (always pronounced "four-twenty," never "four hundred and twenty") came Is it 4:20 already? to be an accepted part of the argot within that group of about a dozen pot smokers, beginning as a reminder of the time they planned to meet to light up, 4:20 p.m.
(It's also worth noting that 4:20 PM is the time of LSD creator Albert Hofmann's first deliberate ingestion of the substance, in 1943 on what was later dubbed Bicycle Day; perhaps this is where the San Rafael teenagers picked up the idea.)
On 1 January 2004 the Governor of California signed that state's Senate Bill 420 which regulates marijuana used for medical purposes. This bill comes years after the term '420' was associated with marijuana and indeed its number likely was chosen because of the existing pop culture connection. This is the tail wagging the dog, not the other way around.