Whereas our concepts of a day, a month and a year are all tied to natural (created) phenomena, the week is directly related to the creation itself. God created for six days and then rested on the seventh - a week. This has been passed on through cultures for thousands of years. The Jews, following the Old Testament laws, regarded the seventh day (now known as Saturday) as the Sabbath. However, early Christians soon made the first day of the week the Lord's day, based all that had happened on the first day: the Resurrection, Pentecost, the Great Commission, the Ascension. This became Sunday, the Lord's day, and became the day they chose to be together to worship. So we are commanded to keep the Sabbath, a day of rest, which for us is Sunday. The question is, are you convinced in your own mind what you believe about this day? It isn't a legalistic thing. Are you giving a care?