The period when the woman is niddah is a time of physical distance between the couple even though the couple remains emotionally close. Immersion in a miqveh, associated with purity, symbolizes the purity of the sexual relationship which is conducted according to the insights of the Torah.
1. After her seven clean days, a woman immerses in a kosher mikveh that evening. She should immerse after sunset of the seventh day, such that by the time she gets home it will already be seit hakokhabim (lit. stars' appearance, fully night). A woman should not immerse during the day of her seventh day. However, if she did (bedi'abad) it counts. If she immersed before the seventh day it does not count. (Taharat Habayit v. II, 14:4, p. 467)
2. If her husband is in town, a woman should not delay her immersion beyond the set time, so as not to delay the misvot of intimacy and of procreation. (T.H. v. 2, 14:2, p. 445)
3. A woman may immerse on shabbat (Friday night) even if she could have immersed earlier in the week.
4. On Friday night, in a heated mikveh, it is preferable to immerse before seit hakokhabim. However, if a woman could not do so, it is permitted to immerse after this time. (T.H. v. 2, 14:3, p. 452)
5. A woman who did not immerse the night following her seventh clean day should immerse the next evening after sunset. If a hardship circumstance prevents her from going at night, such as when she is ill and going at night is difficult, and surely in cases of general hardship for the whole city, such as when women are afraid to go at night because of fear of thieves and muggers or because of cold, or... when there are pits and obstacles near the mikveh and there isn't adequate light and they are afraid of falling in a pit or well, in all these cases it is allowed to immerse during the day of the eighth or a later day, even early in the day. (Translated from T.H. v. II, 14:5, p. 492)
6. If a woman's grown sons are at home in the evening, and if she goes to the mikveh at night they will sense it, and she prefers to conceal her going from them, she may delay her immersion to the eighth day such that she would immerse in daytime and they won't sense her going. (Translated from T.H. v. II, 15:6, p. 547)
7. The above also applies to a woman who desires to conceal her going to mikveh from nosey neighbors curious about her behavior in matters of modesty. Such cases are considered hardship circumstances and she is permitted to immerse during the day of the eighth or a later day. (Translated from T.H. v. II, 14:5, p. 492)
8. Under very pressing circumstances (sha`at hadehaq) _ for example when a woman will not be able to immerse at the proper time after sunset and her husband is leaving for an extended trip that night or they are going somewhere where she will not be able to immerse the next day or the day after _ she may immerse on the seventh day before sunset, and then only shortly before sunset. In such a case she should not return home (not to be in private with her husband) until the stars come out (seit hakokhabim) (T.H. v. II, 14:5, p. 492 and T. H. Haqaser v. II, 14:11, p. 50).
9. A bride may immerse on her seventh day during the day when the wedding is scheduled for that night (T.H. v. II, 14:4, p. 467).