Niddah, Chapter 9, Mishnah 11. This mishnah continues the chapter's discussion of the veses, the fixed pattern by which a woman's period arrives. A woman who has such a pattern must take it into account when the expected day comes around, and she and her husband separate on that day. The question our mishnah addresses is what happens when the pattern moves: the day she had always expected passes quietly, and the blood appears on a different day instead.
The opening case concerns a woman whose blood had reliably appeared on the fifteenth of the month, and then arrived on the twentieth instead. The mishnah puts it in terms of her having been "lemudah", trained by experience, to see on "yom chamishah asar", the fifteenth day, and then having shifted, "veshinsah", seeing on "yom esrim", the twentieth. How does she conduct herself in the coming month? The ruling is "zeh vezeh asurim": neither date may be treated as safe. The fifteenth still binds her, since that remains her established pattern, and the twentieth now demands caution as well, since that is where the blood actually turned up, so she and her husband keep apart on both dates.
The second case: "Shinsah paamayim leyom esrim" - the shift has now repeated, and she has seen on the twentieth twice, while her original expectation remains the fifteenth. Here too the ruling is "zeh vezeh asurim". Two occurrences are still not enough to settle the matter in either direction, so she must continue to be concerned for both days and separate from her husband on the fifteenth and on the twentieth alike.
The third case: "Shinsah shloshah paamim leyom esrim" - she has now changed to the twentieth three times. At this point "hutar chamishah asar", the fifteenth is released and is no longer a day of concern for her, "vekavlah yom esrim", and the twentieth has become her newly established veses.
The mishnah then supplies the rule from which all three of these rulings flow. One side of it: "she'ein ishah kovaas lah", that a woman does not fix for herself a "veses", "ad shetikbaenah", until she has established it, "shloshah paamim", three times over. This is exactly why the twentieth had not yet become her day after a single appearance, and not even after a second one. The other side of it: "ve'einah mitaharas min haveses ad sheteaker heimenah shloshah paamim", she does not become clear of a pattern she already holds until it has been pulled out of her on three occasions, meaning that the anticipated date came and went with no blood three separate times.
Once these two halves are set side by side, the whole mishnah opens up. A veses is built by three, and a veses is dismantled by three. In the interval, while the new day has not yet been built up and the old day has not yet been fully uprooted, she carries both: the day her body once promised and the day it has recently chosen.