Etymology From Middle English *happenynge, *happnynge. By surface analysis, happen + -ing.
1
Slang, Of a place Busy, lively; vibrant, dynamic; fashionable.
“[…]San Francisco is not exactly the most happening place, you know.”2005, Wendy Lawton, Less is More, page 13:
When the show ends, the circular, sunken floor is one of the more happening dance clubs in town.2006, Eliot Greenspan, Neil E. Schlecht, Frommer's Cuba, page 165:
1 more example
They're a little noisy if you open your balcony door but otherwise offer a glimpse of one of Disney World's more happening places.2011, Bob Sehlinger, Menasha Ridge, Len Testa, The Unofficial Guide Walt Disney World 2012, page 157:
2
Slang, Of a person or product Trendy, up-to-the-minute.
Every show has its hottest, most happening trends. If I were to isolate just one for hardware and one for software, this year's hot hardware would be the 386 machines, and the happening software would be desktop publishing.1987 November 16, Steve Gibson, “Desktop Publishing, 386-Based Machines, ‘Happening’ Trends at Comdex”, in InfoWorld, page 42:
Going strong since 2006, this ephemeral nightclub (it's open only for 50 nights each year, in July and August) has become the hottest ticket in DJ land, a combination of the most happening names in music and its spectacular setting at the heart of the Palais des Festivals.2009, Nicola Williams, Oliver Berry, Steve Fallon, France, Lonely Planet, page 883:
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San Antonio is the newest, tiniest, and most “happening” wine appellation in Chile, with just four boutique wineries that focus on quality, not quantity, producing fine pinot noir, sauvignon blanc, and syrah.2011, Nicholas Gill, Christie Pashby, Kristina Schreck, Frommer's Chile & Easter Island, unnumbered page:
happening
noun
Etymology From Middle English happenyng, equivalent to happen + -ing. As term used in performance art popularized by Allan Kaprow.
1
Something that happens.
Three follow-up singles, all ecological or religious in theme, did chart in the lower reaches of the Hot 100. Thereafter, Ocean remained a Canadian happening.1990, Wayne Jancik, The Billboard Book of One-Hit Wonders, →ISBN, page 271:
2
A spontaneous or improvised event, especially one that involves audience participation.
And it was some kind of happening / Some kind of happening / Salvador Dali's garden party1989, “Salvador Dali's Garden Party”, in Privilege, performed by Television Personalities: