Or HaChaim on Exodus 13:14/אור החיים על שמות י״ג:י״ד

1 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.

2The question need not necessarily be exactly "what is this all about?" Any question whose upshot appears to be that the questioner wants to be told what our tradition is all about is to be answered forthwith. It is clear therefore that the Torah does not speak of the night of the Passover but of the commandment to sanctify the firstborn of the womb. When our sages (Mechilta) comment that the four occasions when the Torah cites the sons as asking refer to four different types of children this is all homiletics.