In UK, the tradition of washing on Mondays dates back to the days of wash coppers, dollies, washboards etc. when washing clothes for a family was an all day task. Before there was soap powder one of the jobs was to make flakes by shaving a block of soap. There was also blue and starch solutions to be made up and a heap of other tasks. In those days most people had a roast for Sunday lunch and there would be meat left over to make a simple cold meal such as cold meat and pickles on the Monday. As you would not have had a fridge it made sense to use up the leftover meat the next day. So, as the only day the housewife could get away with not cooking a meal, Monday was the obvious choice for doing the washing.