What was "late" about Moses' return from Mount Sinai? At the end of 40 days, as Moses had said to them, he was supposed to return. The people, however, counted the day he went up the mountain as the first day, but that day did not start the previous night so it was only a partial day and did not count. This is what left room for Satan to do his work.

This is Rashi's explanation. It is based on the Midrash and sounds very farfetched because it does not seem to have any source rooted in the text.. That is unusual for Rashi, who tries to be literal. What made him do this?

And then he writes that the sorcerers who left Egypt with the Israelites are the ones who made the Golden Calf? Why did Rashi write this?  

Dedicated to the memory of Martha Tekuziner, z"l.