It could allow for 1:1 and n:m (small n, small m) messaging on these days, but nothing bigger. Email would be carved out as an exception.
If it were to be enforced, it'd have to be levied onto companies from the top. They'd be forced to engineer these time window blocks.
Maybe every other day would be a social media day.
I think about this occasionally. It would probably be good for all of us as a society. Totally impractical, would get tons of push back and would never happen, but a fun thought.