How do we know that Shabbat in on Saturday?

Hello Rabbi I hope you are doing well. My question is about the sabbath day. Is Saturday the exact 7th day that God rested, or there is room that it's observed on a Saturday as a tradition and there could been a break in the 7 day week cycle and adjustments of the calendar in history. Also, can the Rosh Hashanar help to clarify the accuracy of this?

Shalom!

Thank you for your question.

The Jewish people have been observing Shabbat, the Sabbath, even before they left Egypt, and by extension, even before the Torah was given. It is one of the oldest and most sacred observances in Judaism

The Jews were give a "secret code” of sorts for them to know exactly when was Shabbat.

In Exodus 16 we are told that the Jewish people survived on Manna. Manna was a heavenly bread that would fall from Heaven each day and it sustained the Jewish people during their forty years of wandering in the desert. But here’s the key: The Manna would fall every day except for Shabbat! On Fridays, the people would gather a double portion of Manna so that they would have what to eat on Shabbat. This is why the Jewish people always begin every meal on Shabbat with two loaves of bread. It reminds us about the double portion of Manna, which was bread-like, that the Jewish people collected before Shabbat began.

There is also a Midrashic teaching that goes something like this: "If the non-Jews will come to you and ask, “Why do you observe Shabbat on this particular day? You shall answer them, that we know this day is to be observed as Shabbat because on this day the Manna does not fall".

This went on for 40 years and the seventh day, Saturday, became well established as the God-appointed say of rest.

There is also a passage in Rabbi Yehuda Halevi’s 12th century work "The Kuzari" which points out that the the vast majority of the world keeps a seven-day week which clearly shows that the idea is ancient. In fact, it might have been Adam and Even who invented the seven day week! Adam and Eve sinned in the Garden of Eden by eating the forbidden fruit. This took place on a Friday. They were then banished from the Garden of Eden and rested on the next day. We are told that they continued this pattern for the rest of their life and passed it on to their children: six days of work, and then rest on the seventh day, which is now known as Saturday.

Shemot Rabba 25:11.

Kuzari 2:20