none said:

Not immediately obvious, and we're still trying to figure it out, but when didEmpire last show theatrically, in an almost original form? Our initial guess was that it didn't get shown after the Jedi re-release. But searching afterwards we did find a 1985 poster which was: http://cinemasterpieces.com/cinestarwars.htm
"Produced to promote an exclusive one-time showing of the entire Star Wars trilogy in 9 theaters across the U.S. on March 28, 1985 to benefit the Corporation of Public Broadcasting."

I went to a trilogy screening in late 1987 that was tied to the 10th anniversary and we were told similar screenings were happening in several big cities.