There was also an unspoken rule that no public school sporting events would be held on Wednesday, which was midweek church night.
The change happened in 1975. I was working with a youth organization when the public schools mandated Wednesday after school sports practices. Hundreds of kids dropped out of the program to play sports. Then the Y caved in.
Many organizations never made a comeback after that, and now soccer and other youth sports have filled in the Sunday slot.
mark