סימן קנה סעיף א
תָּנוּ רַבָּנָן, וְהִזַּרְתֶּם אֶת בְּנֵי יִשְֹרָאֵל מִטֻּמְאָתָם, אָמַר רַבִּי יֹאשִׁיָה, מִכָּאן אַזְהָרָה לִבְנֵי יִשְֹרָאֵל, שֶיִפְרְשׁוּ מִנְשׁוֹתֵיהֶן סָמוּךְ לְוֶסְתָּן. וְכַמָּה. אָמַר רָבָא, עוֹנָה. וְעוֹנָה הִיא אוֹ יוֹם אוֹ לָיְלָה. שֶׁאִם זְמַן וֶסְתָּה הוּא בַּיוֹם, אַף עַל פִּי שֶרְגִילָה לִרְאוֹת בְּסוֹף הַיוֹם, מִכָּל מָקוֹם נֶאֶסְרָה מִתְּחִלַּת הַיוֹם. וְכֵן אִם רְגִילָה לִרְאוֹת בַּבֹּקֶר וְלֹא רָאֲתָה, מִכָּל מָקוֹם עֲדַיִן הִיא אֲסוּרָה כָּל הַיוֹם עַד הַלַיְלָה. וְכֵן אִם וֶסְתָּהּ בַּלַּיְלָה, אַף עַל פִּי שֶׁהִיא רְגִילָה לִרְאוֹת בִּתְחִלַּת הַלַיְלָה, אוֹ שֶׁהִיא רְגִילָה לִרְאוֹת בְּסוֹף הַלַּיְלָה, מִכָּל מָקוֹם הִיא אֲסוּרָה כָּל הַלָּיְלָה. וּלְהַרְבֵּה פּוֹסְקִים, לֹא לְבַד בְּתַּשְׁמִשׁ הִיא נֶאֶסְרָה, אֶלָּא גַּם בִּשְׁאָר מִינֵי קְרֵבוּת הִיא אֲסוּרָה. וְכֵן יֵשׁ לְהַחְמִיר.
The Rabbis expounded1Maseches Shavuos 18b. (on the verse) "You shall separate the Children of Yisrael from their impurities."2Leviticus 15:31. Rabbi Josiah said: "This verse is an admonition to the Children of Yisrael, that they must separate from their wives prior to their expected periods." How long before? Rava said,3Maseches Avodah Zara 78a. "An onah." An onah is either a day or a night. If she is accustomed to perceive the flow during the day, even though she is accustomed to perceive this flow at the end of the day, she is nevertheless forbidden to her husband from the beginning of the day. Likewise, if she is accustomed to perceive in the morning and has not yet perceived, nevertheless, she is forbidden the entire day until the night.4Most poskim agree that the length of each onah depends on the length of the day or night at that time and place (Beis Yosef, Taz, Toras Hashelomim and Placey). Chochmas Adam rules that it is no less than twelve hours at any time. Likewise, if her periods occur at night, even if she is accustomed to perceive the flow in the beginning of the night, or if she is accustomed to perceive the flow at the end of the night, nevertheless she is forbidden to her husband the entire night. According to many authorities,5Trumas Hadeshen Responsa 250. not only marital relations are forbidden at this time, but also every other type of intimacy is forbidden, and it is proper to heed this opinion.6Bach, Shach, and Toras Hashlomim say that one who is strict in this will be blessed; but from the Halachic standpoint it is permitted. (Shulchan Aruch, Ramah, Bach, Radvaz etc.)
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)