From the APS website:

“Due to COVID concerns, the UNEXPO has been postponed. The event will be moved to 2022, exact dates yet to come.”

“Scott English and the APS show committee in consultation with me as UNEXPO21 Chairman,” Greg Galetti said in response to a question in the American First Day Cover Society’s Facebook group.

UN Expo 2021 was to be the first in a series of specific-area shows each October at the American Philatelic Center (American Philatelic Society/American Philatelic Research Library) headquarters. Aerophilately 2022 (American Air Mail Society) is scheduled for next year, and there are reports of later October shows at the APC for ESPER (Ebony Society for Philatelic Experiences and Reflections) (black history) and the AFDCS.

It is one of at least five stamp shows scheduled for October that have been canceled, including World Series of Philately shows SESCAL (California) and NOJEX (New Jersey.