Or HaChaim on Exodus 12:41/אור החיים על שמות י״ב:מ״א

1 ויהי מקץ שלושים שנה, It was at the end of 430 yers, etc." The Torah does not tell us what happened at that time, otherwise why would the word ויהי be repeated immediately afterwards. If the Torah had alluded to the time of the Exodus the second ויהי would be superfluous. Another thing requiring exegesis is what painful element is alluded to in the word ויהי in this instance.

2Perhaps the painful element referred to is the very length of time it took for the people to be redeemed. Had they possessed the necessary merit they would not have had to wait until the pre-arranged timetable, i.e. מקץ, the end of the time originally allocated for their bondage. Alternatively, the pain alluded to is the very length of the 430 years mentioned earlier as the time the people sojourned in Egypt.

3ויהי בעצם, it was on that same day, etc. The Torah may use the expression to tell us that though the Israelites departed on that day, the troubles of the Egyptians had not come to an end. We have a reference to the Egyptians being busy on that day burying their dead (Numbers 33,4) whereas at the same time the Israelites left Egypt their heads held high.