Rashi on Leviticus 9:4/רש"י על ויקרא ט׳:ד׳

1כי היום ה' נראה אליכם FOR TODAY THE LORD WILL APPEAR UNTO YOU to make His Shechinah rest upon your handiwork; on this account these sacrifices come (are to be brought) as an obligation for this day (cf. Sifra, Shemini, Mechilta d'Miluim 2 4).