I remember seeing a calendar on my first school year, 1971 in Finland, that marked which Saturdays were school days and which were off. Around the same time, I saw a cartoon about the future labor congress, where a dissatisfied worker asks "Mr President, do we have to every Wednesday?"

From 1965 issue of Time:

> Union-shop steward, emerging from a meeting with management to face a group of workers: “Brothers, we’ve got it made. From now on, we will work only on Wednesdays.”

> Irritated voice from the crowd: “You mean EVERY Wednesday?”

I suspect it’s significantly older than that though.