The earliest specific release date (not just a year, but a month and day) listed in IMDb for Return of the Secaucus Seven is April 11, 1980, at the New York New Directors and New Films Festival. (There is also a vague release date of 1979 in the USA listed.)
I checked the New York Times for April 11, 1980, and the film is indeed indicated as playing at the ND/NF Festival that night. There's a review saying so, an article about the festival saying so, and an ad for the festival saying so.
But the ad for the festival says:
Return of the Secaucus Seven Directed by John Sayles 110 mins. (U.S.A. 1979)
So on the date of the earliest screening I have been able to identify, the festival that ran that screening was promoting the film as having premiered the year before.
I haven't found the answer to your question, but I thought this might be relevant.