They became common extras days off over a period of time to draw potential quality employees to a business.
I'm actually a bit ahead of myself there. That came later. These "standard" days off, medical plans and other perks came from the organization of unions. Go back before large union organization, and working 12-14 days, 6 days a week with no days off other than Sundays, no medical insurance, not even a 10 minute break in the day, was normal. Didn't like it? Too bad. There were plenty of people out of work who'd take your place in a moment, and employers knew that.
All of the common holidays came from union demands, along with shorter work weeks/hours per day, vacation time, and most other perks we now take for granted. In my first full-time job, we got Good Friday off for Easter, paid. But that was only because it was written into the union contract. No job I've had since then includes that day as paid time off. Not unless you want to use one of your vacation days to cover it.
Unions aren't anywhere near as strong as they were some decades ago, and that's mostly because almost all employers offer the types of perks and work schedules unions (literally) fought for without having to be a union shop. If you don't offer similar perks, potential employees you would want to hire will ignore you and go somewhere else.
