Or HaChaim on Exodus 12:15/אור החיים על שמות י״ב:ט״ו

1 שבעת ימים מצות תאכלו, "You shall eat unleavened bread for seven days, etc." This is connected to the fact that the Israelites' dough did not rise due to the suddenness of the redemption. You may well ask how the Torah could legislate (verse 14) a memorial for something that had not even taken place yet? After all, the dough did not rise because of the constant motion of the people carrying it on their backs, something that occurred on the morrow! They had not even slaughtered the Passover lamb yet and already the Torah speaks of an annual memorial to something that had not happened as yet! The whole Exodus would not occur until after the Israelites had slaughtered the Passover and eaten it with unleavended bread, and had thereby acquired the merit enabling G'd to take them out of Egypt! This is the day which G'd would remember for them in order to take them out, etc.

2In support of our contention that the Torah here does not speak of a commandment to eat unleavened bread for seven days, you will note that Pessachim 120 arrives at the conclusion that the mention of "seven days you shall eat unleavened bread" in our verse is not a commandment but a voluntary observance, the commandment to eat unleavened bread applying only on the night the Passover is eaten; the duty to eat unleavened bread became effective only on the evening preceding the Exodus. In later generations, the eating of unleavened bread was not a reminder of a miracle at all. Perhaps the manner in which the Torah reports this legislation is meant to tell us that the eating of unleavened bread on the night of the Passover will be accounted for the people eating it as if they had eaten unleavened bread during all the seven days of that holiday.

3G'd commanded two kinds of מקראי קדש, "holy convocations," one on the first day, i.e. the 15th of Nissan and one on the seventh day, i.e. the 21st day of Nissan. The latter "holy convocation" was in respect of the final stage of the redemption, the drowning of the pursuing Egyptians in the sea. There were two separate miracles which needed to be commemorated then.

4Another reason accounting for the seven days from the 15th to the 21st of Nissan being considered as a unit, i.e. שבעת ימים instead of שבעה ימים, is because G'd did not formulate the prohibition not to eat leavened bread as an outright commandent, such as "do not eat חמץ." I would have thought that a person who does not eat leavened matter during these days is neither culpable nor has he acquired special merit. The Torah therefore implies that not eating חמץ is equivalent to eating מצה and that such a person has acquired the merit of seven consecutive positive commandments (one for each day he did not eat חמץ.) [I believe the author refers to the fact that the prohibition of eating leavened matter in verse 19 is phrased impersonally, i.e. "anyone who eats leavened matter will be cut off from his people," whereas the repetition in verse 20 which addresses itself directly to the people makes no mention of seven days. If this is not the correct interpretation we must assume an uncorrected error in all the earlier manuscripts. Somewhat remarkably, the author does not comment on either verse 19 or verse 20 in this chapter. Ed.]

5The reason the Torah already mentioned seven days here is that the Israelites should be occupied with the performance of מצות right until the moment on the seventh day when the last stage of the redemption would be completed after G'd had displayed even greater miracles than previously.

6There is another mystical reason for the number seven, the root of the souls of the Jewish people being equated with שבעה עינים, as we know from Zachariah 3,9, and the community of Israel is therefore frequently referred to as בת שבעה, "the seven-fold one," compare Zohar volume 3 page 6). [According to Rabbi Moshe Cordovero in his פרדס רמונים the expression שבעת ימים is a reference to the emanation בינה which presides over the 6 days preceding it, i.e. which are subordinate to it. Ed.]