I have the term used on local radio stations out of Toronto, Canada. 
Played as a cue to light up. It seems to have relevance to the ending 
of the work day (I think).

Regards,

Steve Powell
Odessa Pictures, Inc.
View our Demo Reel Online at:
http://odessapictures.com/

On Sep 15, 2005, at 1:03 PM, David Fahey wrote:

> My undergraduate students say 420 is marijuana slang, originating in 
> California, that originally announced that a group would smoke a joint 
> at 4:20.  Anybody with clearer information?
>