advance screening; preview screening
A screening of a movie held before its official release, often for press, special guests, or promotional purposes.
I was invited to an advance screening of the new movie.
Tickets for the preview screening sold out immediately.
試写会 is a more general term for a test or preview screening, often used for feedback before final edits. 先行上映会 specifically emphasizes that the screening occurs before the official release date, typically as a promotional event.
Compound of 先行 (advance, preceding) + 上映 (screening, showing) + 会 (gathering, event).