Steve Rubick, who’s seen the Grateful Dead numerous times, remembers when Timothy Leary came to the Mile High City five decades ago and announced that the Dead would be playing at City Park, just a few blocks from one of Denver's underground LSD labs.

“When [Leary] was asked what he was doing in Denver, he said, ‘I heard there was a shortage of LSD,’” Rubick remembers. “And that was broadcast that night on the evening news. At that time, LSD was legal. They did an acid test in front of the fountain. The Grateful Dead set up right in front of the steps of the Natural History Museum.”

This was on Sunday, September 24, 1967, at the Human Be-In, a celebration of ’60s counterculture (a flier for the event promised “beads, bells, incense, flowers, food and other trippy things”); another Human Be-In had been held at Golden Gate Park in San Francisco earlier that year. The Dead had played its first Denver shows the previous two nights at the newly opened Family Dog. Along with the Dead, the City Park Human Be-In featured an altitude-sick Captain Beefheart, Mother Earth, Tracy Nelson, Lothar and the Hand People and Crystal Palace Guard.

EXPAND The Grateful Dead performing at the Human Be-In at City Park in 1967. Stinkfoot via Flickr

Rubick remembers that it was a beautiful day in City Park, with a lot of naked people frolicking in the fountain and everybody having a great time. The Denver Post reported that about 5,000 people had gathered; “some came to dance, some came to sing, some came to love, some came to watch, and a few came to just be there,” the paper said.

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Near the end of the event, Leary asked everyone to chant the single syllable “om,” which he explained had meant “the sound of the sun, the hum of the seed and the message that the electron murmurs to the proton” for many centuries. According to the Post, Leary also asked the crowd to “look around and see the lawn and grass in City Park is in the state the great process of nature would have it. In other words, tune in, turn it on, drop out, and clean up.”

Today Rubick considers the Dead one of the greatest bands ever, but he was just starting to get into the act back in 1967. “They were kind of a novelty,” he remembers. “The whole scene was just in its infancy back then. The City Park show in Denver, for me, personally, was one of the first kind hippie events that I ever saw.”