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.]