There are manymany discussions of when the fingernails can and cannot be cut according to halacha and minhag. It is very hard to ascertain anything coherent from this information, other than that Friday is the best day to cut them, and that many other days may be prohibited.
Could, though, a woman who needs to cut her nails to prepare for mikvah do so on any day of the week (except Shabbos)?
I'm quite sure the answer is "yes," but I'd like to see a source that actually discusses it.