Going to the mikvah before and after the Temple’s destruction TB Beitzah 18

The Mishnah on TB Beitzah 17b records a disagreement between Beit Hillel and Beit Shammai concerning immersing utensils and human beings to become ritually ready when the Festival begins immediately after Shabbat. “If a Festival occurs directly after Shabbat, i.e., on a Sunday, and one wishes to behave in a proper manner and purify himself and his vessels in honor of the Festival, Beit Shammai say: One must immerse everything before Shabbat, and Beit Hillel say: Vessels must be immersed before Shabbat, but a person may immerse himself even on Shabbat.” (Sefaria.org translation) Repairing an object (תיקון הכלי), which is one of the 39 forbidden categories of work on Shabbat and Festivals, is the reason why people may not immerse their utensils on the Festival. By immersing the ritually unready (טמא) utensil in the mikvah, the person “repairs” its status by making it ritually ready (טהור).

At the very end of the discussion on TB Beitzah 18 the Gemara quotes a baraita. “All who are obligated in immersions immerse in their usual manner, both on the Ninth of Av (Tisha B’Av) and on Yom Kippur, even though it is prohibited to wash on these days.” (Sefaia.org translation) Tosefot ד"ה כׇּל חַיָּיבֵי טְבִילוֹת טוֹבְלִין כְּדַרְכָּן explains why we do not immerse ourselves in a mikvah on Tisha B’Av and Yom Kippur. When the Temple stood the issue was one of being eligible to offer a sacrifice. One had to be in the state of ritual readiness. Ritual readiness (טהורה) and ritual unreadiness (טומאה) was an everyday occurrence and needed to be addressed even on Tisha B’Av and Yom Kippur since sacrifices were offered up on these days as well. Consequently, people were allowed to go to the mikvah on Tisha b’Av and Yom Kippur.

Today now that the Temple no longer stands, we are all in the state of ritual unreadiness. The only person who needs to go to the mikvah according to tradition is a woman after her menstrual cycle so that the husband and wife may return to have sexual relations. Sex is forbidden on Tisha b’Av and Yom Kippur; consequently, immersion in the mikvah is off the table. There is just no need for an immersion.