Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows: 

1. For receiving the Torah.
2. So that Allah speaks with him.

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There are also two opinions concerning the period of this appointment:

1. It was the month of Dhū al-Qaʿdah and the first ten days of Dhū al-Ḥijjah. This is the view of those who say the appointment was in order to receive the Torah.
2. It was the month of Dhū al-Ḥijjah and the first ten days of Muḥarram. This is the view of those who say the appointment was for Allah to speak with Moses.

For what purpose did Allāh appoint a period of forty nights for Moses? When were these forty nights?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The nights are mentioned instead of the days because it is the custom of the Arabs to keep dates according to the night. This is because the beginning of a month is marked by its night, and the Arabs rely on the lunar calendar, as a result of which the days follow the nights (rather than the reverse).
2. Abū Bakr al-Naqqāsh held that the nights were mentioned because Moses was commanded to fast during these days and continue the fast into the nights, so the nights were mentioned. Ibn al-Jawzī comments: This view is baseless.

Why does the āyah mention "forty nights" instead of "forty days"?