May a woman immerse in the Mikveh during Bein Hashmashos of her 7^th day in a time of need, such as to catch a flight and the like?[1]

It is forbidden to immerse during daytime.[2] Accordingly, some Poskim[3] rule that it is forbidden to immerse until actual nighttime, which is Tzeis Hakochavim. Other Poskim[4], however, rule that it is even initially permitted to immerse after sunset prior to nightfall. Practically, one is not initially to immerse until after Tzeis Hakochavim, although in a time of need a Rav is to be consulted regarding immersing prior to this time, after sunset.[5] Whatever the case, Bedieved if she immersed by Bein Hashmashos her immersion is valid.^[6]

[1] See Taharah Kehalacha 21:13 footnote 29; Nitei Gavriel 33:1-2; Pischeiy Teshuvos 197 p. 436

[2] Michaber Y.D. 197:3-5

The reason: This is forbidden due to Serach Bita. [Michaber ibid] This means that there is worry that her daughter will see her mother immersing during the day, and not differentiate between the 8^th day and 7^th day, and when the daughter is married she may come to immerse on the 7^th day. [Rama ibid]

[3] Sidrei Taharah 197:11 [unlike 197:4]; Implication of Pischeiy Teshuvah 197:13 in name of Chamudei Daniel regarding Lechatchila; Darkei Teshuvah 197:7; Chasam Sofer Y.D. 197; Kitzur SHU”A 162:2; Rav Poalim 4:21; Zechor Leavraham 8; Minchas Yitzchak 3:86; Shiureiy Shevet Halevi p. 307

[4] Sidrei Taharah 197:4 and 15 [unlike 197:11]; Chacham Tzevi 11 [Regarding Erev Shabbos]; Zera Emes 78; Pischeiy Teshuvah 197:2 [Regarding Erev Shabbos]; Chesed Leavraham Teumim 1:30; Chut Shani 56; See Darkei Tahara 17 p. 169 that Sephardi women immerse by Bein Hashmashos on Erev Shabbos because of the issue with immersing in hot water.

[5] Nitei Gavriel 33:2; Taharah Kehalacha 21 footnote 29; Shiureiy Shevet Halevi is lenient in a case that she must travel for a long time; See Pischeiy Teshuvah 197:13 in name of Chamudei Daniel regarding Bedieved that is valid

[6] Michaber 197:5 regarding even day of 7^th; Pischeiy Teshuvah 197:13 in name of Chamudei Daniel; Aruch Hashulchan 197:17; Nitei Gavriel 23:3