We are taught that the Mabul began at midday. (Bereishis 7:13) Rashi cites that the reason for this was to demonstrate that, with Hashem's protection, Noach was able to enter the Teivah in front of all the wicked people of his generation and none of them was able to harm him.
Rav Yehonasan Eibshitz zt"l offers an additional explanation that has to do with the way the calendar works. Rav Eibshitz mentions that the historic date which the dove found dry land in the year after the Mabul was the first of the first month and was a Shabbos. If so, then the year preceding this one, the year when the Mabul began, the first day of the first month was Tuesday. (Clearly Rav Eibshitz is using a regualry styled year in which half the months contain 30 days and the other half have 29. The months alternate with 30 then 29 and so forth throughout the year.) Since the first month was a Tuesday then the subsequent month, the month in which the Mabul started, started on a Thursday. Thus, the 17th of that month, the day which the Mabul started, was Shabbos. Since the Gemara teaches that rain on Friday night is a blessing, the rain, instead, began during the daytime as not to give the wicked people even one iota of blessing. (Taanis 23a) Rav Eibshitz continues that, additionally, the mazal of Shabbos daytime is Saturn which symbolizes mass destruction and that was befitting for the beginning of the Mabul. (See Rashi Berachos 59b and the Ibn Ezra's Reishis Chachma) (Tiferes Yehonasan Bereishis 7:13)