See RoseLee Goldberg, Performance Art: From Futurism to the Present (1988) and R. E. Haywood, Revolution of the Ordinary: Allen Kaprow and the Invention of Happenings (1993).
In the later sixties, perhaps due to the depiction in films of hippie culture, the term was used much less specifically to mean any gathering of interest, from a pool hall meetup or a jamming of a few young people to a beer blast or fancy formal party.
Allan Kaprow first coined the term happening in the Spring of 1957 at an art picnic at George Segal's farm to describe the art pieces that were going on. Happening first appeared in print in the Winter 1959 issue of the Rutgers University undergraduate literary magazine, Anthologist. The form was imitated and the term was adopted by artists across the U.S., Germany, and Japan. Jack Kerouac referred to Kaprow as "the Happenings man," and an ad showing a woman floating in outer space declared, "I dreamt I was in a happening in my Maidenform brassiere."
Kaprow’s piece 18 Happenings in 6 Parts (1959) is commonly cited as the first happening, although that distinction is sometimes given to a 1952 performance of Theater Piece No. 1 at Black Mountain College by John Cage, one of Kaprow's teachers in the mid-1950s. Cage stood reading from a ladder, Charles Olson read from another ladder, Robert Rauschenberg showed some of his paintings and played scratched phonograph records, David Tudor performed on a prepared piano and Merce Cunningham danced. All these things took place at the same time, among the audience rather than on a stage. Happenings flourished in New York City in the late 1950s and early 1960s. Key contributors to the form included Carolee Schneemann, Red Grooms, Robert Whitman, Jim Dine, Claes Oldenburg and Robert Rauschenberg. Some of their work is documented in Michael Kirby's book Happenings (1966).
Behind the Iron Curtain, in Poland, artist and theater director Tadeusz Kantor staged the first happenings starting in 1965. Also, in the second half of 1980s, a student-based happening movement Orange Alternative founded by Major Waldemar Fydrych became known for its much attended happenings (over 10 thousand participants at one time) aimed against the military regime led by General Jaruzelski and the fear blocking the Polish society ever since the Martial Law had been imposed in December 1981.