Preparations for Immersion on Saturday Night
Question
Answer
Shalom Rav
According to the Ashkenazi custom, although on Friday the woman is still within the days of mourning, the main rule is to do the preparations on Friday evening and immerse on Saturday night. Only if it is not possible, should the preparations be done on Saturday night.
According to the Sephardi custom, the immersion should be done on Saturday night.
Source
Shulchan Aruch, Yoreh De'ah, Laws of Niddah, Siman 199, Se'if 4
Se'if 4
If the immersion falls on Saturday night, and it is impossible to prepare beforehand, she should prepare on the night of immersion. Gloss: Nevertheless, it is a good practice to wash thoroughly on Friday evening, and on Saturday night to prepare and comb a little (Tur).
Aruch HaShulchan, Yoreh De'ah, Siman 381, Se'if 8 "And if the immersion falls on Saturday night, and on Friday she is still in the days of mourning, and on Shabbat the mourning ends, it is permitted to prepare on Friday.