Rashi on Genesis 8:3/רש"י על בראשית ח׳:ג׳

1מקצה חמשים ומאת יום AFTER THE END OF ONE HUNDRED AND FIFTY DAYS they began to decrease: that was on the first day of Sivan. How so? The rains ceased to fall on the twenty-seventh day of Kislev — you have three days left in Kislev, and the 29 days of Tebeth make 32, and Shebat, Adar, Nisan and Eyar have together 118, making altogether 150.