TT #2 · Parshas Shemini · Aliyah 1 · Passuk 1
Chabad.org — Vayikra Chapter 9 with Rashi
English: On the eighth day Moses called Aaron and his sons and the elders of Israel.
Sifra (Toras Kohanim), Shemini, Mekhilta de-Miluim II 1.
English: “On the eighth day”: we do not know whether this means the eighth day in the count or the eighth day of the month. When the Torah says, “For seven days shall he consecrate you,” we know that it means the eighth day in the count.
Contextual inference — the preceding seven-day consecration establishes the meaning of “the eighth day.”
The phrase “eighth day” is understood as the eighth day of the consecration period, based on the preceding seven days.