I wish I could find more on this. As it stands the reference I did find is is intriguing but apocryphal. Becuase of the year it had to be a guideline for radio and NOT television. TV wasnt availabel to consumers really until 1930.
In the year 1928 The national Association of Broadcasters (NAB) declared that that no advertisements may be aired between the hours of 7 p.m. and 11 p.m. This was "family hour". It was just the sort of gesture that might have actually happened in that window between WWI and WWII. We were building suburbs, and culturally were feeling very wholesome at the time.
But the rule was not to last. The great depression begins on Black thursday. The stock market crashesand for radio outlets the need to sell ads of any kind at any time to continue to existing takes precedence. If it really happened, it would have been then president of NAB William S. Hedges that made the decision... Remember NAB was only formed in 1923 so the ammount of sway they had then is nothign like the multi million dollar army of lobbyists they have now.