San Francisco’s biggest annual cannabis celebration — the 4/20 Hippie Hill — comes with extra splash and dash this year, including a New Year’s Eve-style countdown to a “bud drop,” as well as extra rules designed to keep the open-air party — and the 15,000 revelers expected — in line.

The open-air smokefest occurs just four months after legal, commercial sales of marijuana to adults 21 and over began in California. Recreational use for adults was approved under Prop. 64 in 2016, but it took 18 months for state laws and local licensing to catch up.

While the Hippie Hill event is free and open to anyone 18 and older, attendees are asked to register online in advance and present proof of age and registration upon entry. (The first 5,000 registrants will receive a limited edition Hippie Hill T-shirt; the next 10,000 will receive a commemorative Hippie Hill lanyard.) For the second year in the event’s history, the event will be fenced off, with gates and security, and provide portable toilets, food vendors (in a specially designated “Munchie Land” area) and garbage cans, thanks to official organizers including Sounds Bazaar, Haight Street Merchants and Greenrush.com. The event is not sponsored by the city of San Francisco.

A long list of activities and items are prohibited, including: vending without special event permits; tents, canopies, tables or other structures; coolers larger than 9 by 12 inches in size; barbecues or cooking equipment; amplified sound equipment; generators; glass of any kind; wagons or carts; drones or aircraft; and weapons of any kind.

The event begins at 9 a.m. and officially ends at 5 p.m. It will feature DJs and live music performed on a stage, as well as a video screen beaming a “bud drop” at 4:20 p.m. That’s the time of day that evolved into a code word, 420, for smoking pot, created in 1971 by five San Rafael High School teens who nicknamed themselves the Waldos. The men (now in their 60s)  will launch their first-ever cannabis product, a brand of vape pens in collaboration with Chemistry (with proceeds going to charity) on April 20.

The bud countdown is modeled after the New Year’s Eve countdown and ball drop in New York’s Times Square, said Paul Warshaw, chief executive officer of Greenrush.com, an online cannabis marketplace and a sponsor of the event. Warshaw previously owned Balldrop.com, the largest producer of New Year’s Eve events in the country.

Greenrush is paying what Warshaw described as a “six-figure” sum for production of the 420 Hippie Hill event to keep it free, fun and safe.

“It was an opportunity to give back to the community and an opportunity to make it organized and secure,” Warshaw said. “Social responsibility is a big effort for us, especially since this is the first 420 with adult use.”

In 2017, no major damage was reported at the event. In 2016, year, police made eight arrests, and a man wearing brass knuckles robbed two teens. In 2015, someone hit a park ranger’s head with a bottle, two groups of men robbed attendees, and police reported making five arrests. In 2014, police arrested 11. Fights, traffic jams due to road closures and tons of trash are also reported each year.