My job was advertised as Sat,Sun,Mon evenings. There was also a clause that they could use 60% of the other days as shifts.
In reality, you worked Fri, Sat,Sun because of the 60% rule. Stupid; a person looking for a Fri, Sat, Sun job would not apply because it is not advertised as such and those looking for the days advertised will leave because they change the days. Shortsighted management were unable to accommodate flexibility and you were not allowed to get other work as it may clash with the 60 % flexible working. So you get three shifts but you have to give up 6 days to accommodate them.
short sighted intransient management.