420 has recently been known for the date.  Since the late 60's early 70's was known as a  time of year that festivals, parks, coliseums, auditoriums, have a special day of the year devoted to Cannabis pleasure and medicinal awareness.

Historical review on the internet has discussed an old movie back in 1936, not sure exactly how 420 may relate but its quite the propaganda.  A more recent review from High Times, was generated from a group of high school kids (possibly in a band) started their daily practice around 4:20, a marijuana cigarette was passed to relax from the stressful day, thus 420 and most likely  a few posts in High Times began their tradition.  Another interpretation of 420 may be how long the effect lasts, which of course is an approximate, and it is close.  In 1994 Pulp Fiction referenced 4:20 which was caught by some.  And from the bible Mark 4:20 references seed sown on good soil to produce multiple fold.  While 420 friendly is beginning to be a common phrase, it seems you can place any time in front of the word friendly and it should be known.  520 friendly, as High Times is also trying to give the 9-5 workers a more appropriate time of day to relax from the daily grind. 

PLEASE DON'T SMOKE & DRIVE!