There was TriCon World Science Fiction Convention, at the Sheraton in Cleveland. Gene screened The Cage and Where No Man Has Gone Before. Shatner was there. Harlan Ellison was there. Elizabeth Pickering modeled Andrea's costume from What Are Little Girls Made Of? That would not be seen on tv until October.
Some great stuff on these pages:
galacticjourney.org
cosplayinamerica.com
Some great stuff on these pages:

[September 6, 1966] The Greatest (SF) show on Earth! (1966 Worldcon and Hugo Awards) - Galactic Journey
by Gideon Marcus The Big Show There are many science fiction conventions in the United States, from New York's Lunacon to Westercon, held in San Diego this year! But the granddaddy of them all is the annual Worldcon, which travels from city to city as various fan groups are able to submit a...

TRICON 1966 – Cosplay in America

Tricon : Worldcon : Free Download, Borrow, and Streaming : Internet Archive
Science fiction convention held in 1966. See Wikipedia; Wikidata; SFcentral; Fancyclopedia; Fanac
archive.org