I wrote about the two-hour “Gene Roddenberry Presents: Star Trektennial Special” last month. It was broadcast on August 16th, 1976 by KTLA in Los Angeles and probably syndicated to other stations as well.
Andy e-mailed me this scanned TV Guide advertisement for the special:
Copyright © TV Guide, 1976
If anyone has any details about the “Star Trektennial” special, hit the comments.