Rashi on Numbers 6:10/רש"י על במדבר ו׳:י׳

1וביום השמיני יבא שתי תרים AND ON THE EIGHTH DAY HE SHALL BRING TWO TURTLES — On the eighth day, thus excluding the seventh. Or perhaps this is not so, but it is intended to exclude the ninth day (i.e., that the offering must not be brought later than the eighth day)? But Scripture has fixed a date for the animals which are to be offered (cf. Leviticus 22:27), and has also fixed a date for the persons who bring offerings (Exodus 22:29). How is it in the case of the animals offered as sacrifices? It declared the eighth day and any time from the eighth onward as fit for offering them! So, too, in the case of persons offering sacrifices “the eighth day” implies the eighth and any day from the eighth onwards and only excludes the seventh or any earlier day (Sifrei Bamidbar 29:1).