There is a LOT of misunderstanding on this thread.
A school plans a certain number of school days per year.
They can be of ANY day of the week. It is completely normal for those days to be made up of all weekdays and one Saturday.
The Saturday will be to make it more likely that parents can attend, especially for a sports day, fete or concert.
The Saturday is legally a compulsory school day to the exact same extent as any other school day, if its in the calendar.
You will, if you add it up, find you have one less weekday somewhere throughout the year.
It is normal, it is compulsory, it is advertised in advance, in the annual calendar, same as any other day, the registers will be taken as normal, it will be recorded as an unauthorised absence if you are not there.
To all those people saying it can't be made compulsory, you are quite simply wrong.
The only children I have ever known have their absence authorised on a Saturday school day have been orthodox Jews or seventh day adventists.