베레시트 (창세기) 7:12

ויהי הגשם על הארץ AND THE RAIN WAS UPON THE EARTH — But later on (v. 17) it says. “And the Flood was upon the earth”! But the explanation is this: when He poured down the water at first He made it fall in mercy (gently), in order that if the people would repent, it might prove a rain of blessing; but when they did not repent it became a destructive flood (Genesis Rabbah 31:12) . ‘ארבעים יום וגו FORTY DAYS AND [FORTY NIGHTS] — The first day is not included in this number, because its night (i.e. the night that preceded it, since according to Jewish reckoning a “day” comprises night-time and the following day-time) was not included with it — for it is written (v. 11) “On the same day (i. e. day-time) were all the fountains of the great deep broken up”. (Consequently the forty days ended, according to R. Eliezer, on the 28th day of Kislev (Rosh Hashanah 11b). as the months are counted as regular months, one full (of 30 days) and the next defective (of 29 days), so we have, 12 days of Marcheshvan and 28 days of Kislev.

וַיְהִ֥י הַגֶּ֖שֶׁם עַל־הָאָ֑רֶץ אַרְבָּעִ֣ים י֔וֹם וְאַרְבָּעִ֖ים לָֽיְלָה׃

And the rain was upon the earth forty days and forty nights.