May a Kallah go to Mikveh on the seventh day if that is the day of her Chuppah?
If the seventh day of Nekiyim of the Kallah falls on the day of her wedding some Poskim rule she may immerse during the day, being that she will not be with her Chasan until nighttime. Other Poskim however rule she may never immerse during the seventh day even if the Chuppah will take place at night. According to this opinion she is to immerse at night prior to the Chuppah, or if not possible, then after the Chuppah.
The law if the Chuppah takes place during the day: Many Poskim rule that even according to the first opinion above, it is only permitted for her to immerse during the day if the Chuppah will take place at nighttime, after Tzeis Hakochavim. It does not suffice to make the Chuppah close to nighttime, even if the Yichud will occur at night. Other Poskim however rule that she may immerse during the day even if the Chuppah will take place during the day.
Final ruling and custom: Practically, when the seventh day of her count falls on the day of the wedding, if possible she is to immerse at night prior to the Chuppah and delay the Chuppah until after the immersion. If it is not possible for her to immerse at night and to delay the Chuppah for this long, then she is to immerse during the day but delay the Chuppah until nighttime. If the Chuppah cannot be delayed until night then she is to contact a Rav.
When on the 7th day may she immerse: She may immerse beginning from sunrise. If she immerses prior to sunrise but after Alos, it is valid Bedieved. If she immersed before Alos, then she must re-immerse after sunrise. [Some write that she is to initially immerse after Pelag Hamincha, after completing her last Bedika.]
 197/3; See Yad Efraim 197/3; Taharah Kehalacha 9/16; Nitei Gavriel 1 5/3 and 3 34
 Implication of Rama 197/3, and so learns and rules: Beir Heiytiv 197/8; Sidrei Taharah 197/11; Degul Merivava 197/3; Gilyon Maharsha 197; Panim Meiros 2/4; Pischeiy Teshuvah 197/10; Chochmas Adam 115/14; Rav Akiva Eiger 71; Pardes Rimonim 197/3; Meiy Nida 197/1; Pischeiy Nida p. 67 in name of Rav Yonason Eibashitz; Teshuvah Meahava 2/366; Poskim brought in Yad Efraim ibid; Ben Ish Chaiy Shemini 2/17; Kovetz Zalman Shimon p. 11 suspects for Shach ibid initially although permits in time of need.
 Shach 197/9 in his understanding of Rama; Toras Hashelamim 197/8; Chasam Sofer 189 agrees with Shach; Avnei Nezer 250; Misgeres Hashulchan 197; Tzemach Tzedek 153 seems to suspect for ruling of Shach ibid; Minchas Yaakov [brought in Degul Merivva, Yad Efraim ibid] question the ruling of the Shach although does not argue on him; Yad Efraim ibid concludes that she is to immerse twice, one time before the Chuppah in order to avoid Chupas Nida, and then a second time after the Chuppah. She is not to have Yichud with the Chasan until after her second immersion. See however Beis Shlomo 2/42 who argues against having her immerse twice; See Minchas Yitzchak 4/72
 The reason: As the reason for the prohibition of immersing on the seventh day is not only due to Serach Bita but also due to Shema Tistor Riyasa, and it is thus more severe. [Shach ibid] The Poskim ibid however question the ruling and reason of the Shach based on the fact that if the Chuppah will not take place until night, there is also no chance of Tistor Reiyasa, so why is it any more severe than Serach Bita. See however Toras Hashelamim who defends the ruling of the Shach ibid.
 Degul Merivava ibid; Pischeiy Teshuvah ibid; Rav Akiva Eiger ibid; Rav Poalim 4/21; Poskim brought in Nitei Gavriel 34 footnote 4
 Degul Merivava ibid
 Implication of Sidrei Taharah 197/11; Teshuvah Meahava 197 that so is custom; Mishnas Yaakov 3/128; Meiy Nida 197/1; Poskim brought in Darkei Teshuvah 197/22; Poskim brought in Taharah Kehalacha 9 footnote 26 and Nitei Gavriel 34/2 footnote 6
 Taharah Kehalacha ibid
 As rules the Shach ibid; Kovetz Zalman Shimon p. 11
 Taharah Kehalacha ibid writes that in such a case she is to follow the ruling of the Yad Efraim ibid that she is to immerse twice, one time before the Chuppah in order to avoid Chupas Nida, and then a second time after the Chuppah. She is not to have Yichud with the Chasan until after her second immersion. [See Taharah Kehalacha ibid footnote 25] If even this is difficult for her to perform Taharah Kehalacha concludes that she is to contact a Rav regarding if she may nevertheless immerse by day. [ibid] Nitei Gavriel 34/2 concludes that in a time of need, such as during the long summer days, she may immerse during the day and have her Chuppah take place by day, although avoid having a Halachicly valid Yichud until night. He concludes that so is the ruling that he received from Rabbanim and that is done in practice.
 Chidushei Rav Akiva Eiger 197/3; Pischeiy Teshuvah ibid
 Nitei Gavriel 5/3