There are several types of vestos (patterns in menstrual cycles) that a woman is able to aquire. One such vesses is the vesses hachodesh, which relies on the day of the (Jewish) month. So for instance, if a woman menstruates beginning on the fifth day of the month every month (or for at least three consecutive months), then she has what is known as a vesses hachodesh and must expect her period to begin on the fifth of every month, which entails remaining abstinent on that day and checking herself to confirm the onset of her period.

Now, this all works well with our fixed calendar, with which we know in advance which months contain 29 days and which contain 30. However, in the times when the Sanhedrin was (or will be) active, the months are established by their announcement. I think you know where I'm going with this...

If a woman has a vesses hachodesh which occurs on the first of every month, then she must act as if she will begin her period that day starting already at sunrise of that day. But the Sanhedrin quite often will not announce the new month until later during the day. Or they will not announce it at all, in which case the month begins the following day.

What should she do? Should she treat today (the maybe-Rosh-Chodesh-maybe-thirtieth-day) as her vesses, and if it turns out to be the thirtieth day of the waning month, treat the next day as her vesses as well? Or should she treat today as a normal day until the Sanhdrin declares it as Rosh Chodesh?

[In this case, I would prefer sources over well-reasoned unsourced answers.]

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    I would be very surprised if she treats the day after 29th as normal pending an announcement. You can ask similar questions about all kinds of laws that were only safek relevant that morning (prayer changes, age changes, document dating etc.). Where I thought you were going with this is that women outside of Israel with established vestot hachodesh would need to keep two days of veset because of sefeika deyoma like yom tov. – Double AA Jul 10 '13 at 8:47
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    @DoubleAA, While I would be surprised as well in that case, many Rishonim do hold that prisha b'shaas vestah is only Rabbinical, in which case they may very well hold that she may act l'kula, although there are notable exceptions made for Niddah where we are machmir anyways becuase of the stringent nature of the prohibition. If I had to decide, I would probably compare to a zava min hatorah who was toveles during the day, whose status remains pending until the day closes without her seeing any more discharge. – jake Jul 10 '13 at 9:07
  • @DoubleAA, I am curious, however, how women in chutz laaretz behaved in the times of the Sanhedrin. I suppose the answer to that question is related to the answer to this one. – jake Jul 10 '13 at 9:08
  • "I would probably compare to a zava min hatorah who was toveles during the day" I was thinking the same thing. – Double AA Jul 10 '13 at 9:09

Not a full answer - but too long to format nicely in a comment:

See Tosafot איתמר in Nida 64a

דחזאי ריש ירחא וריש ירחא כו' ואפילו שאחד מלא ואחד חסר ואין הימים שבין ראייה לראייה שוין מ''מ קביעות החדש גורם

It would seem that if one had a Veset as you describe, one would be in doubt until finding out when Rosh Chodesh was declared.

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