At 4:20 p.m., thousands will converge on Golden Gate Park to light up.
MAP: Where is weed legal?
Last year, 15,000 people descended on Hippie Hill, which is now known as Robin Williams Meadow.
This year's event will be the first time recreational pot use is legal in California.
Officials say they neither condone nor sanction the pot holiday, but know it's not going away.
VIDEO: Preps underway for Bay Area first known as 'Canna-Crawl'
"Love it or hate it, the historic 4/20 festival is not going away," said Supervisor London Breed.
Authorities say they're working with a private sponsor to pay for fencing, security and park maintenance.
VIDEO: San Francisco's 2017 4/20 celebration runs smoothly with no arrests
Merchants partnered with the city last year to help with vendors, security and cleanup.
The partnership will continue this year with merchants raising $200,000 dollars for more porta potties, trash cans and security.
Click here for more stories, photos, and video on cannabis.
HeadsUp: This afternoon #SFMuni routes near #GoldenGatePark, the 7, 7X, 33, 37, 43, & NX, will have reroutes. Other service routes in the area may see delays. For complete service details: https://t.co/WF6eBOE46U pic.twitter.com/NbYqpfZnVv— SFMTA (@sfmta_muni) April 20, 2018
We understand that some people see today as more than just another Friday. But if you're planning to celebrate #420day, please do so responsibly. Don't get behind the wheel of a car if you're partying, arrange for a sober driver. #DriveHighGetaDUI pic.twitter.com/5KlNcMZ6FP— SantaClaraCoSheriff (@SCCoSheriff) April 20, 2018
We know that #420Day is a day of celebration for some. We want to remind you that if you PUFF PUFF...PASS THE KEYS to a sober driver. If you’re planning on sparking designate a sober driver now or use a ride-share. @lyft is offering #420 discounts. Be smart. #FridayFeeling pic.twitter.com/CtRpRUo21Q— Sunnyvale DPS (@SunnyvaleDPS) April 20, 2018
Drinking isn’t the only way you can get a DUI while driving. Do not get behind the wheel if you are under the influence of any drugs or alcohol. It could cost you more than just a ticket! #DriveSober #420day pic.twitter.com/SxYjqtWYnG— Campbell Police (@CampbellPolice) April 20, 2018