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SAN FRANCISCO, CA - MAY 13: (R) Mark Chandler, Director, San Francisco Mayor's Office of International Trade and Commerce presents proclamation to (L-C) Anthea Hartig, Executive Director, California Historical Society and Melinda Franklin, San Francisco Travel incoming board chair and United Airlines at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at SFO on May 13, 2017 in San Francisco, California. (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

SAN FRANCISCO, CA - MAY 13: (R) Mark Chandler, Director, San Francisco Mayor's Office of International Trade and Commerce presents proclamation to (L-C) Anthea Hartig, Executive Director, California Historical Society and Melinda Franklin, San Francisco Travel incoming board chair and United Airlines at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at SFO on May 13, 2017 in San Francisco, California.  (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

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SAN FRANCISCO, CA - MAY 13: General atmosphere in celebration of the 50th anniversary of the Summer of Love at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California. (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

SAN FRANCISCO, CA - MAY 13: General atmosphere in celebration of the 50th anniversary of the Summer of Love at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California.  (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

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SAN FRANCISCO, CA - MAY 13: Passengers pose at the Love Bug cutout in celebration of the 50th anniversary of the Summer of Love at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California. (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

SAN FRANCISCO, CA - MAY 13: Passengers pose at the Love Bug cutout in celebration of the 50th anniversary of the Summer of Love at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California.  (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

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SAN FRANCISCO, CA - MAY 13: Dogs from the Wag Brigade are in attendance in celebration of the 50th anniversary of the Summer of Love at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California. (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

SAN FRANCISCO, CA - MAY 13:  Dogs from the Wag Brigade are in attendance in celebration of the 50th anniversary of the Summer of Love at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California.  (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

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SAN FRANCISCO, CA - MAY 13: Madame Tussauds figures of Jerry Garcia and Jimi Hendrix are displayed at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California. (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)

SAN FRANCISCO, CA - MAY 13: Madame Tussauds figures of Jerry Garcia and Jimi Hendrix are displayed at the 'Flowers In Your Hair Day' Celebration with the San Francisco Travel Association, United Airlines and San Francisco International Airport at San Francisco International Airport on May 13, 2017 in San Francisco, California.  (Photo by Steve Jennings/Getty Images for San Francisco Travel Association)