Mon Nazir 16

The Gemara tries to find how does R' Yosi find three days strait of Ziva to make a Zava Gedolah, if we say that part of the day is like the whole day.

Rashi says we cannot say that she saw three days at night, because then we would say that her being clean part of the night is considered as if she was clean that day too.

Tosfos says , this cannot be. We see explicitly, that if she went to the Mikvah on the 11th day of Ziva at night, and have relations that night and sees bloods afterwords, she's Chayiv. We don't say that being clean part of the 11th night is if she was clean the whole 11th day.

Therefore, we must say that we don't consider being clean part of the night as if she was clean the whole day. That Halacha we only say when it starts daylight. However, even if she saw at night, when it becomes dawn, we consider her clean right away for that day. So you still cannot get three consecutive days.

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