Thousands of people will descend on San Francisco's Golden Gate Park to smoke pot for the annual 4/20 celebration, in what may be the last year marijuana is illegal in California.

Fans of the drug have long marked April 20 as a day to roll weed or munch on pot-laced brownies — especially at 4:20 p.m. — and call for increased legal access to it. Crowds with gather in states with legal recreational pot and those where voters and lawmakers are considering it.

In California, this year's unofficial pot holiday could be the last that users have to call for legalization, with an initiative expected on the November ballot. The drug's use for medical purposes got approved in 1996.

Voters in Nevada, Arizona and Massachusetts also are expected to consider marijuana legalization measures. And the Vermont Legislature is discussing a proposal to legalize the possession of up to 1 ounce.

Recreational use already is legal in Colorado, Washington, Oregon and Alaska.

The 4/20 pot holiday that some say has its roots in the San Francisco Bay Area will bring more police, park rangers and other officials this year to make sure it's safe for the 15,000 revelers expected to flood the park's Hippie Hill, Board of Supervisors President London Breed said.

"Because we, as a city, welcome folks from all over the world, we are doing everything we can within our capacity to keep the community as safe and as clean as possible," Breed told the San Francisco Examiner.

The unsanctioned event costs the city between $80,000 and $100,000 per year because agencies are called in to help ensure safety, control heavy traffic and collect trash. Crews have cleaned up more than 5 tons of trash in previous years, Breed said.

The origins of the number 420 as a code for marijuana are murky. Some say 420 was once used by Southern California police to denote marijuana use.

But others say the number became a code in the 1970s among high school students in San Rafael, north of San Francisco, who used it as a meeting time to gather to smoke marijuana after school.