The fourth commandment (the third by Roman Catholic reckoning) is to "remember the Sabbath to keep it holy." The "Sabbath" was for those who originated this commandment the seventh day of the week, actually beginning on what we call Friday evening and ending about twenty-four hours later. Christianity somehow dropped the seventh day as the Sabbath and instituted the first day of the week and put on it all of the restrictions that previously applied to the seventh day. This again is strictly a religious rule, although certain jurisdictions in the United States have had "blue laws" requiring that no business activities be conducted on Sunday. Modern commercial practices are putting these to rest.
So, to put up the "Ten Commandments" in our public schools as either a set of rules to be followed or as a modedl for laws for our secular society is extremely inappropriate. They would, if required to be followed, establish religious practice and observance, quite at odds with the First Amendment of the Constitution.
