The tenth commandment clearly states that the Sabbath should be kept on the seventh day and we know through the Jewish people that the seventh day of the calendar is Saturday on the Roman calendar. Also, note that in the Spanish language the word for Saturday is "sabado" and not that in the English language. The word "Sunday" would mean the day of the Sun. Why is it that mainstream Christianity keeps Sunday of the holy day of worship?
It is the fourth commandment of the Ten Commandments which required the Israelites to set the seventh day of the week aside as a holy day. "Remember the sabbath day, to keep it holy. Six days you shall labor and do all your work, but the seventh day is a sabbath of the LORD your God; in it you shall not do any work, you or your son or your daughter, your male or your female servant or your cattle or your sojourner who stays with you. For in six days the LORD made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the LORD blessed the sabbath day and made it holy" (Exodus 20:7-11). In recounting the laws, Moses stated this: "Observe the sabbath day to keep it holy, as the LORD your God commanded you. 'Six days you shall labor and do all your work, but the seventh day is a sabbath of the LORD your God; in it you shall not do any work, you or your son or your daughter or your male servant or your female servant or your ox or your donkey or any of your cattle or your sojourner who stays with you, so that your male servant and your female servant may rest as well as you. You shall remember that you were a slave in the land of Egypt, and the LORD your God brought you out of there by a mighty hand and by an outstretched arm; therefore the LORD your God commanded you to observe the sabbath day" (Deuteronomy 5:12-15).
Therefore, we learn that the Sabbath day was to be a day of rest that illustrated two things for the Israelites: It recalled that God created the world and rested on the seventh day, and it reminded them that they were once slaves in Egypt, but God delivered them and gave them rest. Please take note that for most of the world, the second point holds no significance. Only the Israelites were slaves in Egypt. My ancestors were not slaves in Egypt. However, this is perfectly fine because the laws brought to the Israelites by Moses were only meant for the Israelites. "Then Moses summoned all Israel and said to them: "Hear, O Israel, the statutes and the ordinances which I am speaking today in your hearing, that you may learn them and observe them carefully. The LORD our God made a covenant with us at Horeb. The LORD did not make this covenant with our fathers, but with us, with all those of us alive here today"" (Deuteronomy 5:1-3). Moses then begins to recite the Ten Commandments.
In case you still have doubts, just go back one chapter in Deuteronomy when Moses introduces the topic of Israel's laws. "Or what great nation is there that has statutes and judgments as righteous as this whole law which I am setting before you today?" (Deuteronomy 4:8). The laws given to Israel were exclusive to Israel. No other nation had laws like them. The Ten Commandments were a vital part of these laws. Just a few verses later we read, "So He declared to you His covenant which He commanded you to perform, that is, the Ten Commandments; and He wrote them on two tablets of stone" (Deuteronomy 4:13). Hence, we learn that the Old Testament, including the Ten Commandments, was a law intended for the nation of Israel. The article "Why We Don't Follow the Old Testament" explains in detail why Christians are not under this old law.
The reason why Christians worship on the first day of the week (i.e. Sunday) is quite simple. This is the day on which Christians have always worshiped. In Acts 20:7 we read: "Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight." What was notable about the first day of the week? It was the day when the disciples came together to partake of the Lord's Supper. The first day of the week was also the day when contributions were collected. "On the first day of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come" (I Corinthians 16:2).
While the day of rest, the Sabbath day, held significance to the people of Israel, the first day of the week holds special meaning to Christians. It was on the first day of the week that our Lord rose from the grave. "Now when the Sabbath was past, Mary Magdalene, Mary the mother of James, and Salome bought spices, that they might come and anoint Him. Very early in the morning, on the first day of the week, they came to the tomb when the sun had risen. ... Do not be alarmed. You seek Jesus of Nazareth, who was crucified. He is risen! He is not here. See the place where they laid Him" (Mark 16:1-2, 6; see also Luke 24:1; John 20:1). And the Lord's church was established on the day of Pentecost, which always falls on the first day of the week (Acts 2:1ff). Hence, the first day of the week became known as "the Lord's Day." "I was in the Spirit on the Lord's Day, and I heard behind me a loud voice, as of a trumpet" (Revelation 1:10).