The noun happening is sort of the same as its synonyms: incident, episode, affair, and case. At first definition, it is something that happens; an occurrence, or an event.
But at second remark, the true magic of happening reveals itself, “an unconventional dramatic or artistically orchestrated performance, often a series of discontinuous events involving audience participation.”
And the third noted explanation expresses that happenings are any events considered worthwhile, unusual, or interesting.
The Brits, oh the lovely Brits, happening: an improvised or spontaneous display or performance consisting of bizarre and haphazard events.