Niddah, Chapter 9, Mishnah 10. The preceding teaching established that a woman's period can announce itself through consistent physical sensations - yawning, sneezing, abdominal pain, chills and the like - and that three occurrences fix such a sign as a veses. Our mishnah now asks a sharper question: while those sensations are running their course, is she to be treated as one who has already begun to bleed?
Everything hinges on which stage of the episode her flow tends to arrive. The mishnah's first case is the woman who is "lemudah", practiced and habituated, at seeing "bitchilas havestos", at the opening of the veses. Concerning her the ruling is that any tahor item she handled "besoch havestos", during the stretch of time that the sensations occupy, is pronounced "temei'os". Her known pattern is that blood shows up right along with the onset, so we must reckon with the strong likelihood that it already emerged and escaped her notice. The second case is the woman whose flow appears only "besof havestos", at the tail end of the episode. For her, objects of taharah that came through her hands while the sensations lasted stay "tehoros", since in her case the beginning of the symptoms carries no presumption at all that bleeding has yet started.
Rabbi Yosei now adds a further dimension: "af yamim vesha'os vestos", days and also hours qualify as vesos. A cycle, that is, can be pinned down not only to a date but to a precise point on the clock. His example is a woman whose fixed time of seeing falls "im hanetz hachamah", with the rising of the sun. Rabbi Yosei rules that "einah asurah ela", she is forbidden to her husband at that moment and no other. Throughout the whole night leading up to it she remains tehorah and permitted to him; when the sun rises they separate; and once that moment has passed with no blood appearing, she is permitted to him for the remainder of the day, right up until sunrise returns the next morning.
"Rabbi Yehudah omeir: kol hayom shelah" - Rabbi Yehudah disagrees and holds that the whole day belongs to her veses. In his view the separation falls on the night that precedes her established time, so that the night before sunrise she is asurah to her husband. Once sunrise arrives and she has seen nothing, she is permitted to him for the entire day that follows, until nightfall of that evening.