A woman's initial monthly discharge renders her a niddah. Seven days after that she may immerse in a mikveh and be again permitted to her husband.
The next eleven days after that are potential “zivah” days. Any discharge during these days renders her a “zavah.” If the discharge lasted for one or two days, she is called “minor zavah” and needs only to wait for another day free of discharge, and can go to a mikveh. However, three days of discharge render her a “major zavah.” She needs to observe seven clean days, immerse in a mikveh, and in the time of the Temple bring an offering. After that, the new cycle begins, where the next discharge will render her a niddah.
This is the Torah law, but in later generations, due to the difficulty of observing the day counts of niddah and zivah, the custom has become to treat any discharge as requiring seven clean days after it.
Eruvin 66 – Giving away
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