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Disclaimer: I am not a religious Jew, this question is only out of curiosity.

I understand that observant Jews observe Shabbat from Friday night to Saturday. And from this question and this it seems that it is important that this happens on the correct and accurate time of week.

My question is, how do you know that the Saturday that everyone is using is correct (as in, exactly 7*x days after God's rest day)? Is there any record on who began declaring that Monday is Monday, etc. until it is used by today's civil calendar? And how can we know that the sequence is never broken? (for example, the sequence of calendar time of year was broken when there is a switch from the Julian calendar to Gregorian calendar)

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The shift from Julian to Gregorian didn't remove days but dates. But there have been other calendar reforms that changed the number of days in a week, discussed in comments on the Samoa question. – Monica Cellio Jan 2 '12 at 15:49
Fitri, thanks for your interesting question and welcome to the site; I hope you stick around end enjoy it. – msh210 Jan 2 '12 at 17:56
@Monica Cellio: Right. In the Gregorian calendar reform of 1582, Thursday, October 4 was followed by Friday (not Monday), October 15. – Daniel ben Noach Jan 2 '12 at 18:43

There is no way to know for certain.

However, there are a few indications that what we have the "correct" day.

Firstly, there is the halacha that if you do not know what day it is, you keep Shabbat on the 7th day from the last time you kept shabbat. So even if the calendar did a shift, presumably the Jewish people would have shifted the name of the correct day.

Say, for example that next year the world Calendar people decided to move the week 3 days. Then from that time on, "Shabbat" would be on Tuesday, and not on Saturdays. In other words, there is no direct connection between Saturday and Shabbat, except for the fact that they currently coincide.

Now, Historically, the first Shabbat was only kept after the Israelis left Egypt, and we are told by Moses that this day is to be the day of rest. We must assume, but we do not know for a fact, that Moses told us the correct day according to Gd's will.

Lastly, even today in Israel, the days of the week are named "day one, day two, day three, day four, day five, day six, Shabbat" They are not given any other names.

The aspect of your question regarding when Saturday became Saturday, and Sunday, Sunday is a good one for the History stack exchange, but I don't think would be well answered on the Judaism one.

One last point. In America, some calendars show Monday as the first day of the week, and other calendars show Sunday as the first day. It is interesting to me, that on a Christian theological level, Monday should be the first day, and Sunday the last day. Also from a secular, "business" perspective, Monday should be the first day and Sunday the last. However for Jewish people, Sunday being the first day of the week is the most natural. My point here, is to restate what I said earlier, that what name we give the 7th day is not as important as the fact that it is the 7th (i.e. Shabbat) In Israel, Sunday is a workday and the first day of the week.

After looking some things up on Wikipedia, I noticed that the ISO sets Monday as the 1st day of the week, and Sunday as the 7th. So indeed the "world Calendar folks" have said that the 7th day is Sunday, and yet Jews still keep the 7th day (Shabbat) on Saturday.

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do we have unbroken records from Moses' Shabbat to today's one? – Fitri Jan 2 '12 at 16:05
@Fitri, yes. Not written records necessarily, but Shabas (the sabbath) has been observed, week in and week out, ever since his time, always seven days after the previous Shabas. (Even when it couldn't be observed, due to, for example, pogroms, Jews kept track of when it was, or at least Jews in other parts of the world did.) – msh210 Jan 2 '12 at 16:18
@Fitri while there are records of some mitzvot being kept, or not kept during various historical periods, there is no period which does not mention keeping shabbat in some form or another. – avi Jan 2 '12 at 16:24
Firstly, there is the halacha that if you do not know what day it is, you keep Shabbat on the 7th day from the last time you kept shabbat. Only for Kiddush, not for Melacha. One has to avoid Melacha every day as it's a Safek DeOraisa – Shmuel Brin Jan 21 at 3:14

We do not determine the date of the Sabbath by looking at the calendar and seeing which day is Saturday (or Friday night). Rather (much like my answer to the Samoa" question) the Sabbath is seven days from the previous Sabbath. We have a living mesorah, tradition, on which day is the Sabbath, which happens to coincide with what it commonly known as Friday night to Saturday night. This is one area that illustrates that the observance the Torah commands of us cannot really be reconstructed from the text alone but requires the living community to transmit the practices to the next generation. Nevertheless, this is also a area where there is a fair amount of collaboration since the outside world has its "traditional" dating systems which are seldom altered, and when they are it is generally discussed, and discussions exist about when our Sabbath is in comparison with those systems. (Incidentally, Christians specifically consider the day they loosely call the Sabbath to be the first day, not the seventh. They are not disagreeing with what day of the week is the 'seventh'. Since the accuracy of this statement has been challenged in the comments try every single answer (currently) here for example)

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@msh210, See here. – jake Jan 2 '12 at 16:29
@Jake nowhere does that say that they view the day they loosely call sabbath to be the first day. – avi Jan 2 '12 at 16:43
@avi, If you call the day that they take off for mass and prayer "the day that they loosely call the Sabbath" (which the accepted answer there notes that many Christians do call it "their Sabbath"), then yes it does. The second answer there says that Christians all agree that the "real Sabbath" (the seventh day) is still the civil calendar's Saturday. Just that they celebrate on Sundays, the first day of the week. – jake Jan 2 '12 at 16:49
@avi, That's why they call the Christians who celebrate on Saturday "Seventh Day Adventists". – jake Jan 2 '12 at 16:59
My parenthetical comment is correct and not really debatable. They davka observe the first day in commemoration of their own beliefs which I don't care to elaborate upon here. The history here is clear, you will find ignorant individuals who assume that Sunday is the 7th day because they call it the Sabbath and the Bible says the Sabbath is the 7th day, but this is simply ignorance of Christian thought and history. Period. – Yirmeyahu Jan 3 '12 at 3:32

In Sanhedrin 65b (line 24 and further) Tornosrufus asked same question to R. Akiva. R. Akiva answered that there are three proofs that the day Jew think is Shabbos is a real Shabbos.

  • River Sebation is very fast on other days and on Shabbos it streams slowly.
  • Baal Ov couldn't be applied on Shabbos.
  • Smoke comes out of the grave of Tornosrufus's father each day except for Shabbos.

So we can see that, at least at R. Akiva's days (~2000 years ago), the sequence hadn't been broken.

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It is interesting, too, that there is a graffito from Pompeii (dated 60 CE) - analyzed here - that gives a day of the month and a day of the week (though with the unusual twist of considering the night and day to each have their own weekday designations). The information given matches our present-day calendar. So we see that already then - about ten years before the destruction of the Second Temple, and during R. Akiva's lifetime - a continuous weekly cycle was already in use. – Alex Jan 17 '12 at 0:48
This is a miraculous claim. Can anyone verify that Sebation river indeed travels slower at sabbath? Tornsrufus? Who is he? I'll look that up. – Jim Thio Nov 24 '12 at 13:52
tornos rufus sentence akiva to death. Out of all the graves, why only his graves emit smokes and is this verifiable? – Jim Thio Nov 24 '12 at 13:54
@JimThio in the answer I didn't claimed that the proof is verifiable today. According to "why only his grave emit smoke?" - you are welcomed to ask a separate question about that. – jutky Nov 27 '12 at 7:06

Seeing as all religions hold that Saturday is the seventh day and no one disputes as to when Saturday is, it stands to reason that the days of the week have been handed to the while world generation after generation. Otherwise, wouldnt there be a cult somewhere who argue???

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See here for various cults that view other days as a "day of rest" other than Saturday. en.wikipedia.org/wiki/Sabbath – avi Jan 2 '12 at 13:20
Sorry, what I meant was that everyone agrees that Sunday is the start of the week even if not the day of rest – Yehuda Jan 2 '12 at 14:06
@Yehuda if you read the article I linked to, you will see that not everyone agrees that Sunday is the first day. For example the ISO does not. – avi Jan 2 '12 at 16:26
Thanks everyone, checked your link Avi, your right, I will stick to quoting from now on! Great to meet everyone, hope i quoted enough in my Haneiros Halolu answer. Thanks for the tips. – Yehuda Jan 2 '12 at 16:47
Yehuda, welcome to the site. You can edit your answer to clarify what you meant, add links, etc, by clicking on the "edit" link below your message. – Monica Cellio Jan 3 '12 at 3:21

Civil Saturday/traditional Shabbat is not correct Shabbat day. The proper way to observe Shabbat is to follow the Moon cycles. Some Jewish (Philo) and non-Jewish commentators mention this opinion.

This is because Bereshit 1:14 says that Moon is for "appointments" and Shabbat is one of the "appointments" (holy meetings - miqra qodesh - Vaikra 23).

Vayikra 23:15 tells us that complete weeks are called "perfect shabbaths" and incomplete weeks are called "non-perfect". This is because Sinodal lunar month is 29.5 days so there are incomplete weeks. This means that a month is 4 complete weeks and 1 incomplete week.

Also, it would be strange that God would not provide us with the sign for His holy day of rest, which is why we should rely on the moon phases to identify Shabbat.

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Note Gen 1:14 does not specify which of the solar bodies has which function. – Double AA Jan 21 at 1:30
@DoubleAA It does. its Bereshit 1:16 – Aleksandr Sigalov Jan 21 at 2:19
So the first Sabbath was not on a Sabbath (3 days after the moon was created)? – Double AA Jan 21 at 2:45
When the Torah allowed on 15 Nissan "that which is needed to eat" אך אשר יאכל לכל נפש does that overrule the fact that it's Shabbat too? Why did it even specify not to do work on 15 Tishrei, if that was already Shabbat? What about the Korbanot Musaf for Shabbat in Num 28? Why aren't they mentioned in the sections there about what to bring on 15 Nissan and 15 Tishrei? Doesn't it seem from all that that the Torah doesn't think that every 15 Nissan and 15 Tishrei is Shabbat, but rather that Shabbat is independent of those dates (unlike what you claim, that they were always Shabbat)? – Double AA Jan 21 at 2:57
So there are 5 Sabbaths each month? 1-8-15-22-29? How does that possibly fit with "six days shall you work and on the seventh rest" ששת ימים תעבד וביום השביעי תשבת? Your claim is getting more ridiculous as this goes on. It's almost like you've never read the verses. And if you think it's corrupted, then you can make anything fit: just say anything that disagrees is corrupted. (Of course, no one should take claims like that seriously...) – Double AA Jan 21 at 4:04

protected by Shmuel Brin Mar 13 '15 at 6:43

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