Rashi on Deuteronomy 9:18/רש"י על דברים ט׳:י״ח

1ואתנפל לפני ה׳ כראשנה ארבעים יום AND I FELL DOWN BEFORE THE LORD, AS AT THE FIRST, FORTY DAYS, as it is said (Exodus 32:30) “And now I will go up to the Lord, perhaps I may atone [for your sin]”. At that ascent, which was the second I made, I tarried there forty days — consequently these terminated on the twenty-ninth of Ab, since he had ascended on the eighteenth of Tammuz. On that same day in Ab He became reconciled with Israel and said to Moses (Exodus 34:1) “Hew thee out two tablets”. He stayed there another forty days; consequently these terminated on the Day of Atonement (the tenth of Tishri). On that same day, the Holy One, blessed be He, was gladly reconciled (i.e. completely reconciled) with Israel and said to Moses, I have forgiven according to thy word. On this account it was appointed for pardon and forgiveness. And whence do we know that He was reconciled with them with complete goodwill? Because it is stated in respect to the forty days of the last tablets (in respect to third period of forty days) (Deuteronomy 10:10), “And I stayed in the mountain according to the first days”! What was the case with the first days? They were passed in God’s goodwill! (for they had not yet committed the sin of worshipping the golden calf). So, too, the last forty days were in God’s goodwill! You must now say (admit) that the intervening forty days were passed in God’s anger (Midrash Tanchuma, Ki Tisa 31; cf. Rashi on Exodus 18:13).