here is answer for you..

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.

Also, 420 is the penal code section for marijuana use in California.
Nope. Section 420 of the California penal code refers to obstructing entry on public land. The penal codes of other states list different entries for 420, but none of them matches anything having to do with marijuana.

However, 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.