There are 613 Mitzvot in the Torah. Why do you think those ten Mitzvot (actually more like 14) are special? Who told you that the commandment to keep holy the Sabbath day was more important than the commandment not to mix wool and linen?
Answer: the Christian Church did, and really, this was long before the Reformation, so let's just be honest and say it was the Catholic Church that did it. Why? Because those commandments clearly codify the basic natural moral law that all humans are expected to follow. Murder didn't become wrong because God said so in the Ten Commandments, it was wrong all along (Cain, call your office ...). Same for adultery and stealing.
However, there's an exception. (Actually, there's two, but let's keep it simple.) Nothing in natural law tells you that Saturday is the day to keep holy, and not Thursday or maybe the second and fourth Tuesday of every month, or whatever. That is why the church determined very early on that, while natural law can argue that time ought to be sanctified by, e.g., setting aside a day every week for the worship of God, the specific setting aside of Saturday was ceremonial law given to the Jewish people only, not to everyone.
Which is why Christians have gathered for worship on Sundays since before the last apostle was cold and stiff. It's just a fact. It's not "Babylonian" or any garbage like that. It's common Christian practice to meet on the first day of the week, the day when Our Lord was raised from the dead.
Despite the papal decree, many Catholics were quite aware of the pagan roots of their calendar. Cardinal John Henry Newman discussed this in The Essay on the Development of Christian Doctrine, published in 1878.
“The rulers of the Church from early times were prepared, should the occasion arise, to adopt, to imitate, or to sanctify the existing rites and customs of the population, as well as the philosophy of the educated class. The use of temples and those dedicated to particular saints, and ornamented on occasion with branches of trees (wreaths), incense, lamps, candles, votive offerings on recovering from illnesses, holy water, holy days and seasons (the entire Church calendar), use of calendars, processions, blessings on the fields, sacerdotal vestments, the ring in marriage, chants, the Kyrie Eleison — are all of Pagan origin, and sanctified by adoption into the Church.”
Many of the names of the months and days of the week have their roots in paganism: • Janus: named for the god Janus. It was originally the 11th month.
• February –named for the pagan Roman festival of purification. It was originally the 12th month.
• March – named for the god Mars, the Roman god of war. This was originally the first month.
• April – named for the Greek goddess of love, Aphrodite. The latin aprilis indicates a time of fertility.
• May – this month is named for the Roman goddess Maia, a deity of growth and increase.
• June – named for the goddess Juno, the wife of Jupiter, also known as the Queen of Heaven.
• Sunday – this day is in honor of the sun god.
• Monday – this day is in honor of the moon goddess.
• Tuesday – this day is in honor of the Norse god of war, Tiu, or Tyr, the son of Odin, or Woden.
• Wednesday – this day is in honor of the chief Norse god Odin, or Woden.
• Thursday – this day is in honor of the Norse god of thunder, Thor, who is the oldest son of Odin.
• Friday – this day is in honor of the Norse goddess Frigg, She is the goddess of darkness, goddess of the sky, and goddess of love. She is the wife of Odin. Saturday – this day is in honor of the Roman god Saturn.
The Gregorian calendar is based on the solar cycle, putting it at odds with the Hebrew calendar which is lunisolar, established by witnesses reporting to the Sanhedrin upon seeing the new moon.
It is important to note that the Roman calendar that preceded the Julian calendar observed an eight-day week called the nundinal cycle. This put it directly at odds with the seven day week described in the Bible as embedded in the creation of the world. And the Sabbath was moved from the seventh day to the first day, Sunday, named for the pagan sun god.
The Mishnah (oral law) identifies four distinct new-year dates: The 1st of Nisan is the new year for kings and festivals; the 1st of Elul is the new year for the cattle tithe… the 1st of Tishri is the new year for years, of the years of release and jubilee years, for the planting and for vegetables; and the 1st of Shevat is the new year for trees—so the school of Shammai; and the school of Hillel say: On the 15th thereof. Rosh Hashanah 1:1
The Bible refers to the holiday of Passover as falling on the 15th day of the first month. This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you. Exodus 12:2
Since Passover is required to be celebrated in the spring, it should fall around, and normally just after, the vernal (spring) equinox. Most Jews recognize the first day of the month of Tishrei, described in the Bible as the seventh month, as the New Year.
The tradition of beginning the New Year on January 1 was based on the Roman calendar and remained so in the Julian Calendar though by the Middle Ages, some countries began the new year on December 25. It is believed this mid-winter New Year was based on its proximity to the solstice, a pagan celebration of the sun god’s rebirth. Naming the first month after the pagan god Janus was appropriate since he was depicted as having two faces, one looking forward and the other looking back. Scholars have suggested that the drunken parties that characterize New Year’s celebrations have their roots in orgies meant as a form of service to the pagan gods.
Christianity perhaps inaccurately adopted December 25 as the birthday of Jesus and January 1 as the seventh day after his birth and therefore the day of his circumcision. The Council of Nicaea in 325 CE been formally ratified theologically that day as symbolizing the supersession of the Church over the Jews as God’s chosen people.
At the same time, Pope Sylvester, whose sainthood day and the day he was buried is commemorated on December 31, convinced Roman Emperor Constantine to prohibit Jews from living in Jerusalem. At the Council of Nicaea, too, Sylvester promulgated a host of new anti-Semitic legislation. It is for this reason that January first was frequently marked throughout history by the burning of synagogues, Jewish books, and other acts of Jew-hatred. Ironically, New Year ’s Day is known in Israel as Sylvester.
One such anti-Semitic New Year’s Day event took place in 1577 when Pope Gregory XIII, who established the Gregorian calendar, decreed that all Roman Jews had to attend a Catholic conversion sermon given in Roman synagogues after Friday night services. The penalty for not doing so was death. Then, on New Year’s Day of the next year, the Pope signed a law forcing Jews to pay for a “House of Conversion” whose purpose was to convert Jews to Christianity. Three years later on New Year ’s Day, he ordered the confiscation of all the Roman Jewish community’s Hebrew scrolls and books.