Reference for Niddah 138:2

אלא אידי ואידי שב לנדה ותמני לזיבה בלילותא תמני לנדה בעי

and if she appeared before us at night we allow her eight in respect of menstruation<span class="x" onmousemove="('comment',' On the first night of her arrival and on the following six nights immersion is necessary because each might be the night following the seventh day, while on the eighth immersion is required on account of the possibility of the discharge having appeared on the very night of her arrival which caused the day following to be regarded as the first of the prescribed seven days of menstruation. ');">²</span>

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