David Grudt did some more research on that Keep U.S. Beautiful special we've been talking about. Sez he, it did indeed air on 3/27/73. On the West Coast, it followed the Academy Awards — this was back when they aired on NBC — which started at 7 PM and were supposed to conclude in time for a Bob Hope Special at 9 PM and Keep U.S. Beautiful at 10. The Oscars, of course, ran over…by 38 minutes so the two specials (and the 11 PM news and The Tonight Show) were delayed accordingly. On the East Coast, Keep U.S. Beautiful was on at 8 PM and Hope at 9 PM and the Oscars started at 10:00.
Some years later, the Academy and whichever network was then airing them decided it wasn't a good idea for the most important awards to be presented when for so much of the country, it was after 12:30 in the morning so adjustments were made. Thanks, Dave.