Answer: The weighty evidence of the true Sabbath of the Creator will be discovered once again under the rubble of error. This evidence clarifies that His ordained and appointed dual action system of both the “sun and moon” were to rule together in the heavens and measure time for “all four” units of time a specified in Genesis 1:14 KJV. Yahuwah ordained at Creation this dual action of both the sun and moon, to beacon signs (Sabbaths complete, also known as weeks), seasons (months), days, and years (luni-solar years). This calendar is known as the Hebrew luni-solar calendar, lunar for moon and solar for sun. The current Roman Gregorian calendar is reckoned on solely the "sun" in direct defiance of the creator's appointed dual action system. This pagan system of time keeping and worship was then named "sol", meaning one or only, and is where we get the term solar today. Any unit of time that is a “fruit” of this counterfeit time keeping system is therefore spurious, and is a product of a calendar with a false origin, making neither Saturday nor Sunday the true Sabbath of Scripture.
It all began with Julius Cæsar in 45 B.C., just a few years before Christ Yahushua was born. However, this Roman calendar was not observed in Israel until they lost their sovereignty as a nation beginning after the destruction of Jerusalem.
In the mid-1st century B.C. Julius Cæsar invited Sosigenes, an Alexandrian astronomer, to advise him about the reform of the calendar, and Sosigenes decided that the only practical step was to abandon the lunar calendar altogether. Months must be arranged on a seasonal basis, and a tropical (solar) year used, as in the Egyptian calendar. “The Julian Calendar,” Encyclopedia Britannica.