Sforno on Exodus 32:21/ספורנו על שמות ל״ב:כ״א

1הבאת עליו חטאה גדולה, a reference to the display of merriment in serving the golden calf. Moses criticised Aaron for having proclaimed the feast which led to this rejoicing over the golden calf instead of instead of being a feast for G’d. On account of this he was forced to entreat G’d for an additional measure of mercy for His people. This is also what he meant when addressing the people saying: אתם חטאתם חטאה גדולה “you have committed an exceedingly great sin,” (verse 30) Also, when appealing to G’d, he commences by saying that the people had been guilty of an exceedingly serious sin (verse 31). This theme returns once more when G’d speaks of עון-פשע-וחטאה, sins of three different levels of seriousness in Exodus 34,7. In 34,9 Moses begs for forgiveness for two of these categories of sin, עון and חטא, not daring to ask for forgiveness for פשע, deliberate taunting of G’ by one’s sin. Moses. who had shown understanding for the fact that Aaron had been under great pressure in acceding to the making of the golden calf, cannot understand why he lent his hand to heaping additional sin on the people by having declared a public holiday. He had not been under pressure to do that and should have known that the people would understand this as an invitation to celebrate the new leader, replacing Moses.