60 years ago, in the merchant navy, my old man got Sundays at Sea as either an extra days leave or an extra days pay. That was when Saturday working was the norm.
How many Sundays do you work OOA and how much leave do you get on return!
Yes, you are paid 7 days but will they pay for your leave on a 5 day week basis (good) or 7 days (bad).
The civil service is paid salary for a limited hour, 5-day week, and like the military a weeks leave - Fri-Sun is 5 days. If you are paid in lieu of leave though you only gt 1/365ths of your annual salary.
Don't know how civil firms handle it.