It used to be the case that folk on Unemployment Benefit (UB) COULD work on a Sunday without it affecting their benefit because the benefit week ran from a Monday to a Saturday. Various benefits were classed as weekly benefits or daily benefits that were paid weekly. Some benefits were classed as a seven-day benefit or a six-day benefit.
Some benefits are classed as overlapping, meaning you cannot get the full amount of both and for any period that overlaps and one benefit is reduced by a proportion of the days involved. Don't want to make it any more complicated but years ago, some benefits were reduced by 1/7 or 1/6 for each day of the overlap depending on the other benefit involved.
Legislation was changed bit by bit (I can't mind on whether Jobseeker's Allowance was a six or seven-day benefit when it was introduced) so that all the benefits are now seven-day benefits and that closed the loophole of folk being able to work and earn anything on a Sunday without affecting their UB or JSA.