Aug. 28-29, 1976 - Personal Computing Conference in Atlantic City. Read all about it on pages 6-7 of the October 1976 DDJ. Trenton show came before this, and there was a midwest event in between Trenton and AC, but AC was more of a national conference vs. hobby event. This was a lecture topic at VCF East 7.0. Here's the video: https://youtu.be/8P_I5H_9uvU.