Question:
In the Bible it says to rest on the seventh day of the week, because G‑d created the world in six days and rested on the seventh. Today Christians rest on Sunday, Muslims rest on Friday, and Jews rest on Saturday. How do you know that you are celebrating on the right day?
Answer:
Your very question was dealt with by the Midrash Rabbah, a compilation close to 2,000 years old.
We were first commanded to keep Shabbat in the desert shortly after we left Egypt. How did we know when to keep it? In Exodus 17, we are told that during our journey in the desert, manna fell every day except for one—the Shabbat. So what did we eat then? A double portion fell every Friday so that we would have what to eat the next day as well. On the first Friday after the manna began to fall, the people were surprised to see so much manna—double that which they had received on each of the past five days. When they came to ask Moses about this phenomenon, he revealed that the next day would be the Shabbat and that no manna would fall at all.
The actual wording of G‑d's message to Moses and the Jewish People is "See that G‑d has given to you the Shabbat." The Midrash points out that the word used is "see" and not "know." It explains:
This is what G‑d was saying to them: "If the idolaters will come to you and ask, 'Why do you make the Shabbat day on this day?' you will tell them, 'See, the manna does not fall on the Shabbat.'"
For the next 40 years we had a weekly reminder of the Shabbat every time the manna did not fall. Since then, we have continued to keep count and will continue to do so for the rest of time.
Rabbi Yehuda Halevi in his 12th century classic, "The Kuzari," points to an earlier source for the universally accepted week. He points to the striking fact that the vast majority of the world keeps a seven-day week—evidence that this must be a very ancient custom indeed. How did it begin? When Adam was banished from Eden on the first Friday afternoon of Creation, he rested that first Shabbat. He then counted six days and again rested on the seventh. Ever since, his offspring in many parts of the world have emulated this practice, living their lives by a seven-day week.
