On a sunny, unseasonably warm winter day in 1967, an estimated 20,000 to 30,000 people participated in what would become a preview of the Summer of Love. On January 14, hippies and other counterculture participants swarmed to the polo grounds in San Francisco’s Golden Gate Park in a gathering called the Human Be-In.

Timothy Leary advised attendees to “Turn on, tune in, drop out.” Bands including the Grateful Dead and Jefferson Airplane played and poets like Allen Ginsberg and Lawrence Ferlinghetti made appearances. A parachuter—rumored to be famous chemist Owsley Stanley III—gave out LSD. Hell’s Angels acted as the event’s security.

“People sat on the grass with nothing to do, sometimes moving up near to the small platform where a poetry-reading might be going on, or where a band might be playing. There was no program; it was a happening,” wrote researcher Helen Perry, who attended the event and likened it to a religious rite.

The “happening” was an effort spearheaded by Allen Cohen and Michael Bowen, who were among the founders of the underground newspaper the San Francisco Oracle, to bring together various cultural and political groups in what they called “a gathering of the tribes.” In the lead-up to the event, the underground newspaper the Berkeley Barb wrote, “Berkeley political activists are going to join San Francisco’s hippies in a love feast that will, hopefully, wipe out the last remnants of mutual skepticism and suspicion.”

The Be-In’s organizers based their plans off a smaller event the previous fall called the Love Pageant Rally, which was held to mark the day California outlawed LSD. That event saw several thousand people attend in San Francisco’s Haight-Ashbury neighborhood and Golden Gate Park, but the Be-In’s organizers wanted to achieve something even bigger.

“There was an awakening going on, and we knew it was happening across the country, and we knew there were pockets of people out there who felt isolated and alone and scared,” Martine Algier, who helped publicize the Be-In, told the San Francisco Chronicle on the event’s 50th anniversary. “We wanted to send a signal out to them: ‘Hey, it’s OK to come out and spread your wings.’”

Send a signal they did: Mass media covered the successful Be-In, showing youth all over the country that San Francisco was the epicenter of the counterculture. “Retrospectively, I feel quite certain that the Be-In also marked the beginning of nationwide attention,” Perry wrote.

Young people began to pour into San Francisco after the event. “The crowds on Haight Street got bigger and bigger and bigger,” Rusty Goldman, a 1960s historian and archivist who was at the Be-In, told the Chronicle. “Word spread like fire. Everybody came to San Francisco.”

A February article in Newsweek called what was happening “a psychedelic picnic.” As more people began to move to the Haight-Ashbury neighborhood, countercultural leaders realized they needed to organize for the influx and created a council. In April, the council held a press conference urging America’s youth to come be part of what was happening in San Francisco that summer—it was time for the Summer of Love.