Every year when April 20 rolls around people across the U.S. celebrate an unofficial holiday: weed day.

Stoners everywhere recognize 4:20 as being the time of day to light up, and when the date is 4/20 it's time to kick back, light up and smoke all day -- especially at 4:20. If you're into that kind of thing, of course.

Now that several stats have legalized marijuana to various degrees plenty of folks from sea to shining sea will be lighting up this year, some even in public. Colorado, which has the loosest regulations on pot, sees thousands gather in downtown Denver -- aka the Mile High City -- for a pro-marijuana legalization rally. But the 2013 party is the first since the state legalized the substance for recreational use.

But where did the term '420' come from? Well, many attribute it to an old police code for people caught smoking pot, but that's just a myth.

The actual story goes back to a group of friends in San Rafael, Calif., who called themselves the Waldos (because they hung out against a wall outside their school -- no, really). In 1971 the friends planned to find a rumored abandoned cannabis crop they heard about and would meet at 4:20 each day. They never found the weed garden, but since then the term has become a generalized term for pot smoking.

Plenty of Hollywood stars have since openly professed their love of the leaf, so here are Hollywood's 13 biggest potheads, all of whom have admitted in the past they enjoy smoking the ganja. Start after Willie...