Reggae music played loudly and the smell of weed pervaded the air at the annual marijuana-ingesting event.
The origins of the number 420 as a code for marijuana are murky. The most accepted version is rooted in a public high school in Marin County.
There, a group of 1970s high school students say they coined the number as code for the time to meet after school to toke up.
Fans of the drug have long marked April 20 as a day to roll weed or munch on pot-laced brownies and call for increased legal access to it.
At least 24 states and U.S. territories, including Colorado, Washington, Oregon and Alaska, have legalized some form of marijuana use.
The issue is expected to appear on the ballot in more than a dozen states this November, including California, Nevada, Arizona and Massachusetts.
The Associated Press contributed to this report