Not a historian of work, but my understanding is that one point where 9-5 started was Henry Ford when he realised that longer working days led to a higher turnover of workers in a tight labour market. There's an interesting podcast that touches on it here: https://www.ppfideas.com/episodes/where-are-we-going%3F-the-...
The push for the 8 hour working day started many decades before Ford. He was the first major employer to argue it benefited employers too, but many smaller employers had accepted it before that.
May 1st as an international day for Labour demonstrations, for example, started in 1889 after a proposal from what is now the AFL-CIO to resume the fight for the 8 hour day in commemmoration of the Chicago Haymarket massacre.
Like I say, not a historian, so thanks for the pointer.