Its because builders merchants are open on a saturday, to allow people who during the week are working to do their building work…a shelf here, a shed there, etc etc…i dont agree with working weekends full stop, but many fodd supply chains now insist that their work begins on a thursday and ends on a monday…to squeeze every weekend out of you… I have never believed supermarkets should be open on a sunday, it even says that in our bible ( except for motorbike shops apparently ) 
but what a godsend it can be !!..guess its ok to open shops on a sunday, so long as its other thats working eh !! myself included.