Zavim, chapter 1, mishna 2, in my own, free translation, following the consensus of major commentators:
Suppose someone has a seminal emission on the third day of his count of seven clean days after zov. Bes Shamay say that that cancels the two days he's counted, so he must begin the seven days anew. But bes Hilel say that it only ruins the day itself, so he must count five clean days thereafter. Rabbi Yishmael says that, even according to bes Hilel, if the emission was on the second day, then the first day is canceled, and he must begin the seven days anew.
Why the difference between the second and third day (according to Rabbi Yishmael's interpretation of bes Hilel)?
