Sforno on Deuteronomy 16:3/ספורנו על דברים ט״ז:ג׳

1לא תאכל עליו חמץ שבעת ימים, even though in Egypt the prohibition of eating chametz was in effect for only one day, as our sages pointed out in Pessachim 96.

2תאכל עליו מצות, seeing it was an offering commemorating the redemption, something that took place in a short moment in time, it is not appropriate to eat chametz, something which by definition requires time in order to become leavened. (compare Hoseah 7,4, “from kneading the dough until it has been baked”).

3לחם עוני, bread which they ate while in a state of ענים, external pressure, not having time to wait until the dough had risen so that it became leavened. The taskmasters had always been rushing them, during their entire stay in Egypt as slave labourers.

4כי בחפזון יצאת, the reason why we have to remember this state of haste is because haste is a form of pressure. It was because you always had to rush while being slaves that the redemption when it came also came in the guise of extreme haste. Jeremiah 31,12 describes such an experience in the words והפכתי אבלם לששון, “I will turn their mourning into joy.”

5למען תזכור את יום צאתך מארץ מצרים כל ימי חייך, the reason why I have commanded to eternalize this manifestation of G’d’s direct guidance of your fate, השגחה, is so that the compensation of the shorter lunar year with the seasonal orbits of the solar year be effected through the addition of extra days or an extra month, so that this month will always occur partially after the equinox of spring. I did not legislate anything similar for the solar year as there is no need for it, and that would have little to do with the Exodus. There is no reason why the gentile nations should commemorate the Exodus of the Jews from Egypt. כל ימי חייך, every time the calendar is adjusted, be it to add a day or to add a month, you will be reminded of the reason for this being a reminder of the Exodus from Egypt.