Why does the Torah need to write that" on this very day Noach... entered the Ark"?
Rashi: To teach us that, even though the people threatened Noach that if he dared enter the Ark they would smash it and kill him, HaSh-m protected him from them (by surrounding the Ark with wild animals) and, despite their threats, he entered it in broad daylight. 1
Ramban (to 7:8): Noach, his family and the animals (which are mentioned immediately afterwards) all entered the Ark on that same day, and not before. 2
Rashi writes: "[Noach's detractors wanted to prevent him from entering]; and HaSh-m said, 'I will bring him in [to the Ark] in the presence of everyone!'" What is the source in the Pasuk for this Midrash?
Gur Aryeh: The verse could have stated, "on that day;" rather, it states, "b'Etzem ha'Yom ha'Zeh"-" on that very day" (which implies broad daylight, in full view).
The Torah says that the rain began to fall on the 17th. When did it stop?
Rashi: The rain began to fall in the daytime (as the Torah specifically writes in 7:11). Consequently, the first day is not included in the forty (full days), which only begin on the eighteenth, so it ceased on the 28th [of Kislev, according to R. Eliezer].
Rosh (to 7:17): It began to fall on [the night after] the 16th, so the 17th is the first of the 40 days. It finished on the 27th, like it says in Seder Olam.
Moshav Zekenim: It began to fall on the day of the 17th. From the 18th [of Marcheshvan] until the 27th [of Kislev] are 39 days, and the night of the 28th joined to the day of the 17th to complete 40 full days. 1
רש"י: למדך הכתוב: מהיכן למדנו זאת?
גור אריה: אם לא כן, למה הדגיש הכתוב "בעצם"?!