Rabbi Zeira's stringency
Rabbi Zeira's stringency (Aramaic: חומרא דרבי זירא) or the stringency of the daughters of Israel (Hebrew חומרת בנות ישראל) relates to the law of niddah (a woman during menstruation) and refers to the stringency expounded in the Talmud where an additional five days are added to the Torah-based seven-day niddah prohibition as applicable in Torah law and rabbinic Judaism.
The stringency was enacted due to the confusion of Jewish women between the counting of niddah (i.e. 7 day counting from onset of menstrual flow), zavah ktanah (minor zavah, i.e. one "clean" day of counting) and zavah gedolah (zavah major, i.e. seven clean day counting). Thus, the most stringent of all three, the zavah gedolah state, was accepted as the new norm of minimal tammei period.
The Torah-based menstrual impurity laws were initiated with the reported giving of the Torah at Mount Sinai in the year HC 2448 (-1313 BCE). The stringency was established approximately 1,500 years later, during the amoraic period in approximately HC 3961 (200 CE). The extended lapse in timeline giving way to the stringency is viewed, by some, as being an outcome of the destruction of the temple in Jerusalem and the ensuing scattering and turmoil.
Halakhic status of the stringency
The Talmud Bavli disagrees on how to view the said stringency in terms of it being categorized as a custom or a law, Rava - in dialogue with his student Rav Pappa - abstained from viewing it as an halakha, saying that it is location-based, with communities fully entitled not to comply. Rashi views Rava's comments to his student as hinting that the said stringency evolved into a takanah.
The Rambam (Hilkhot Issurei Biah 11, 3-4) viewed the stringency as is, a "stringency" alone and not a rabbinic enactment, but he nonetheless advocated adherence to it since (by his era) all known communities abided by it, making it as a neder (vow) that is implicitly applicable.
Rabbi Kook's leniency
Among other poskim, Rabbi Avraham HaKohen Kook was of the opinion the stringency is on par with any other rabbinic prohibition, and perhaps lesser than a full-blown rabbinic prohibition. Thus, if need arise to seek leniency from it, leniency may be sought, and applied.
- Talmud Bavli, Niddah p. 67
- Daath Kohen responsa, ch. 84