Pesachim 90 - Just-in-time purity

The time to bring the Passover sacrifice is the afternoon just before the Holiday of Passover. If one is ritually impure in the afternoon but will be pure the next day, that is, when evening comes and the Holiday of Passover begins, they can slaughter a sacrifice for him. Even though he is not pure now, he will be pure when he sits down to eat it at the Seder.

What could be an example of this? A zav (a special kind of male impurity) who experienced two discharges must count seven days without discharges and immerse himself in a flowing spring. After that, he is pure once the night begins. So, that is exactly the case we are talking about if today he counted his seventh day. Even if this zav had three discharges, which obligates him in an additional offering - they can still slaughter the Passover lamb for him. He will be pure at night, and he will bring the additional offering later on, but he can come to the Seder and eat.

Art: Waterfall in Telemark by August Cappelen