1,097. Mikvah on Friday Night
160:4 In extenuating circumstances, if it is not possible for the woman to perform chafifah by day, she may do it at night. She must be careful, however, to do a thorough job and not hurry. Similarly, if it's not possible for her to do so at night, she may do her entire chafifah during the day.
160:5 If the woman's mikvah time fell on Friday night, she performs chafifah by day, being very careful to finish before twilight so that she doesn't come to violate Shabbos. When it comes to lighting the candles for Shabbos, the preferred course of action is, if possible, to go home after preparing herself or to do her preparations at home. After chafifah but before twilight, she lights the candles and then immerses. If this is not possible, her husband should light. If this is also not possible, she should light the candles and recite the brachos while it is still day. Before lighting, she states that she does not accept Shabbos through this act of lighting because such a condition is effective when necessary. However, that which some do, which to recite the bracha over burning candles after they immerse, must be abolished because they are actually reciting a bracha in vain.