What year was the "Summer of Love"?

Flower power, psychedelic music, and free love all collided in San Francisco's Haight-Ashbury district in one summer in the '60s. Hippies from across the country flocked to the city, creating a cultural explosion that would define an entire generation. The media dubbed it the "Summer of Love," but exactly which year did this legendary gathering of peace, love, and questionable fashion choices take place?