Parashas Metzora · Passuk 15:19
Chabad.org — English translation + Rashi
Torah Temima points to Pesachim 90b.
English: “Seven days she shall be in her niddah” — one might think she may immerse while it is still day; Scripture teaches “seven days she shall be in her niddah” — she remains in her niddah all seven.
Full seven days before immersion
Chazal require the niddah to complete the full seven days; she does not immerse on the seventh day while it is still daylight in the manner of a zavah.