Some confusion yet resides among them:

On October 6, 1966, LSD was made illegal and all scientific research programs on the drug were shut down.
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LSD was first synthesized in 1938 and discovered to be psychoactive in 1943. It became popular in the '60's and was made illegal in 1967
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October 6, 1955 in History
Event:
LSD made illegal in U.S.
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Oct 6|LSD made illegal in the US
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LSD was legal in the United States until 1967
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The October 6, 1966 date gets the majority vote, and it jibes with my recollection.
My recollection being taking it, legally, in late September 1966, and seeing it abruptly made illegal two weeks later.
Fred Turner, in a piece excerpted from his book From Counterculture to Cyberculture: Stewart Brand, the Whole Earth Network, and the Rise of Digital Utopianism, in The Edge, glosses the Trips Festival segment of Brand's history a little, kind of like Tom Wolfe did, and partially, I think, from having read and absorbed Wolfe's facile take on all that wild profusion:

Toward the end of 1965, Brand and Ramón Sender Barayón, a composer of electronic music and a friend of USCO's Michael Callahan, thought up the Trips Festival as a way to bring the burgeoning scene together. Together, they found promoter Bill Graham (then a member of the San Francisco Mime Troupe) and hired the Longshoreman's Hall in San Francisco for three nights: Friday, January 21, through Sunday, January 23. By this time, the federal government had outlawed LSD, so posters promised an Acid Test—a full-blown psychedelic experience—without LSD.

Turner writes "According to Tom Wolfe, it was also the start of the Haight-Ashbury era", deflecting the onus for having said that, but getting the line out there anyway.
Conception's notoriously difficult to draw lines on. Whatever that was - era or moment - it was already well alive by then.