Froggequene more or less has it. Originally people worked a seven day week in ye olde days, but this was deemed oppresive and anti-christian so Sunday became the only day off - a day of rest and reflection, and a day of religion. Then in more recent history it was decided that as well as a day of rest and devotion to god, the people should have a day of exercise and following pursuits. Interestingly enough we weren't given the whole of Saturday off. Instead the people of Britain were allowed Wednesday afternoon and Saturday afternoon off, to play/watch sport (hence the naming of Sheffield Wednesday who were originally a cricket team (i think) and only allowed to play Wednesday as they weren't given the saturday afternoon off also), it also explains why football matches traditionally start at 3pm on a Saturday - giving workers change to finish work and get to the match!)
it is only in the latter half of the 20th century that taking the whole of Saturday off became the norm (unless you are a shop worker of course!)