Which Minchah is the Torah referring to?
What are the implications of "D'var Yom be'Yomo"?
Rashi #1: It refers to the Musafim listed in Bamidbar 29:12 - 39, where the Torah prescribes a different Korban for each day of Sukos, including Shemini Atzeres.
Rashi #2: If for whatever reason, the Kohanim failed to bring the specified Korban on any one day, they could not bring it on the following day.
Rashi #3 (in Pesachim, 58a): It implies after midday
Rashi (Ibid.): Since the Torah never uses the word 'Boker' in connection with the Musaf, only 'Yom, as the Gemara says in Yoma, 34a