Why does the Pasuk see fit to repeat the law of Michyah?
Rashi: It comes to include a case where a Nega appeared on one of the 24 tips of limbs 1 which are initially precluded from Tum'ah because the Nega cannot be seen in one go, where the area became fatter and a Michyah appeared on it. 2
Why does the Torah insert the word "uv'Yom"?
Rashi: To teach us that there are some days on which the Kohen does examine a Nega, and some days on which doesn't. The latter incorporates a Chasan during the period of his Sheva B'rachos (both as regards the Chasan himself and as regards his clothes and his house) and all the days of the festival.
QUESTIONS ON RASHI
Rashi writes that the verse teaches about a Nega on one of the 24 extremal limbs that are normally excluded from Nega'im, and it became fatter. What forced him to say so? Simply, the verse discusses one whose body was covered with Tzara'as, and a part became healthy! Toras Kohanim says that if a Nega was in a place excluded from Tzara'as, even if it became fatter, it is Tahor!
Korban Aharon: Rashi did not learn from Toras Kohanim, rather, from the Tosefta (2:12). It says that the 24 extremal limbs are excluded from Nega'im, but if they have a flat area k'Gris, they receive Nega'im. 1
Panim Yafos: Also Rashi discusses when the body was covered with Tzara'as, and a part became healthy. One might have thought that this is more stringent, and a Michyah of any size is Metamei, if the place widened and now it is proper for a Nega; the verse teaches that the same Shi'ur (k'Adashah) applies.
Moshav Zekenim, Da'as Zekenim: The verse was repeated to teach that even though initially a place is proper for Nega'im only if it has a flat area k'Gris, if the body was covered with Tzara'as, a Michyah k'Adashah suffices to be Metamei, so such a flat area suffices.
Panim Yafos: The Tosefta can discuss when from the beginning they had a flat area k'Gris. This is no proof that if a Nega was on a curved place, and afterwards it widened, it is Tamei!
Rashi writes that "uv'Yom" teaches that there are some days on which the Kohen does examine a Nega. In Sanhedrin (34b), it teaches that he does not examine a Nega at night. In Mo'ed Katan (8a), we learn this from "Li" (it appears to me like there is a Nega), and not through my lamp!
Moshav Zekenim: The Gemara often brings the verse that seems to teach the law simply [even if really, it is learned from elsewhere. Really, "uv'Yom" teaches like Rashi says, and we exclude night from "Li" - PF.]