Commento su Esodo 13:14

כי ישאלך בנך מחר WHEN THY SON ASKETH THEE מחר — There is a usage of the word מחר that refers to “now” (i. e. to the period of time that is nearest to “now” viz., to-morrow), and there is another usage of מחר that refers to a day following after the lapse of some time, as, for example, this מחר here and as, for example, (Joshua 22:27) “That your children may not say to our children in time to come (מחר)” which occurs in the chapter about the sons of Gad and the sons of Reuben (Mekhilta d'Rabbi Yishmael 13:14:1).

מה זאת?, a reference to the unusual legislation of having to redeem the firstborn impure animal donkey seeing that its body could not have been sanctified due to its inherent impurity. Similarly, the need to break this animal’s neck as an alternative to redeeming it sounds peculiar to the son who is asking.

והיה כי ישאלך בנך, "It will be when your son enquires of you, etc." The Torah means that when your son observes you perform the ceremony of redeeming a firstborn he will ask you the reason for that commandment. In that event you are obligated to explain the meaning of the commandment. If the son does not ask you, you are not obligated to answer him except on the night of the Passover. This is the reason the Torah added the word מחר, "tomorrow;" This means that even if your son will ask you already tomorrow you are immediately obligated to tell him the reason for this commandment. The Torah adds the word לאמור to indicate that what is meant is a situation when your son expects an answer to his question. If the son, however, simply asks מה זאת, i.e. "what is this all about?" a derisive way of questioning, then you are under no obligation to answer him at that time.