Niddah, Chapter 7, Mishnah 1. This mishnah deals with a woman who sees blood precisely at bein hashmashos, the twilight period whose status as day or night is itself in doubt, and with the confusion that results in her count. It closes with a pointed remark of Rabbi Yehoshua.
Why twilight ruins a count
A woman's status turns on days. The seven days of niddah are followed by eleven days of zivah, in which a sighting of blood carries the potential to make her a zavah, and a woman who has given birth counts her days of purity as well. Everything depends on knowing which day a sighting belongs to. Twilight, which we cannot resolve as belonging either to the day that is ending or to the day that is beginning, therefore leaves such a sighting hanging between two days, and with it the woman's entire reckoning.
The cases listed
The list opens with "haroah yom achad asar bein hashmashos": one whose sighting falls in the twilight of the eleventh day, the hinge at which the eleven days of zivah run out and a fresh period of niddah begins. Four more moments are then named. "Techillas niddah": the twilight that opens her niddah days. "Sof niddah": the twilight in which those days expire. "Techillas zivah": the twilight at the head of the zivah days. "Sof zivah": the twilight in which they run out. In each of them the blood appeared in that unresolved stretch between one day and the next.
Two further cases round out the list: "yom arbaim lazachar veyom shmonim linekeivah." A woman who bore a son and has a sighting in the twilight of her fortieth day, and a woman who bore a daughter and has a sighting in the twilight of her eightieth, stand in the very same difficulty, for those are precisely the days on which her period of purity following childbirth runs out.
"Bein hashmashos lekulan, harei eilu to'os." In every one of these cases the sighting fell at twilight, and every one of these women is left in error: her count has been thrown off and she cannot say with certainty where she stands. The practical results follow from that doubt. She must be concerned that she is a possible zavah, and where a korban is called for, she brings it out of doubt, and it is not eaten.
Rabbi Yehoshua's comment
"Amar Rebbi Yehoshua: ad she'attem mesaknim es hato'os, taknu es hapikchos." Rabbi Yehoshua responds to all this careful work of sorting out the confused ones. Before you invest yourselves in straightening out the women whose doubt comes from twilight, women who genuinely do not know whether their sighting belongs to the day before or the day after, turn your attention to the clear-headed ones.
His point is that there is no shortage of women who know exactly when they saw, with no ambiguity of twilight at all, and whose count is nonetheless in disarray, because their pattern of sightings is so irregular that even with perfect knowledge of the dates they cannot determine when they are a niddah and when they are a zavah. Rather than lavishing all this precision on the doubtful cases, says Rabbi Yehoshua, set right the many women whose facts are certain and whose reckoning is still tangled.