The evolution of the stock market began in 12-13th century Europe, France to be specific, then went on to Amsterdam. Saturday and Sunday were traditionally "Off work" days, moreso Sunday due to the Christian no work policy on the 7th day (universe was created in the first 6 days, and on the 7th, God rested) This is most likely the beginning of it. Also, banks and many businesses, until recently, did not work on Saturday or Sunday.