Just as we have relied on 'man' to hand down to us Yahuwah's Word over the past several thousand years, we also rely on 'men' counting to seven each week for the past few thousand years.
Answer: It is Yahuwah who has preserved His Scriptures. Scriptures states that when the Israelites were unfaithful to Yahuwah, as a punishment He caused them to forget His true New Moons, Feasts, and Sabbath.
“I will also cause all her mirth to cease, her feast days, Her New Moons, Her Sabbaths -- All her appointed feasts,” says Yahuwah. Hosea 2:11
Remember, 490 years of probation was given to the Israelites until Yahushua came on the scene. Then they totally flunked the test of recognizing Yahushua as the promised Messiah, and fulfilled prophecy by putting Him on the cross. After the stoning of Stephen, the gospel then went to the Gentiles and finally in A.D. 70
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.
The Israelites as a organized unit did not recognize their own Creator and rejected Him. Yet you believe that after all that, this same renegade bunch would still carefully protect the true Sabbath, when they couldn’t and wouldn’t protect the true Savior?