John Gills Exposition of the Bible Commentary - Exodus 32:1
And when the people saw that Moses delayed to come down out of the mount ,.... The time, according to the Targum of Jonathan, being elapsed, which he had fixed for his descent, and through a misreckoning, as Jarchi suggests; they taking the day of his going up to be one of the forty days, at the end of which he was to return, whereas he meant forty complete days; but it is not probable that Moses knew himself how long he should stay, and much less that he acquainted them before hand of it;... read more
Matthew Henry's Complete Commentary - Exodus 32:1-6
While Moses was in the mount, receiving the law from God, the people had time to meditate upon what had been delivered, and prepare themselves for what was further to be revealed, and forty days was little enough for that work; but, instead of that, there were those among them that were contriving how to break the laws they had already received, and to anticipate those which they were in expectation of. On the thirty-ninth day of the forty, the plot broke out of rebellion against the Lord.... read more