Or HaChaim on Exodus 12:14/אור החיים על שמות י״ב:י״ד

1 חקת עולם תחגהו, "you will celebrate it as an everlasting statute." Why did the Torah not only use the expression לדורותיכם, "for your generations" which it uses normally? While it is true that the Mechilta states that the word "your generations" would imply only the minimum, i.e. two generations, and the expression חקת עולם means forever, it is still peculiar that the Torah had to use both expressions. The reason is connected to the discussion in Berachot 12 about the verse "in order that you remember the day you came out of Egypt for all the days of your life (including messianic times)." The opposing interpretation of that verse claims that the words "all the days of your life" mean "the whole days of your life" i.e. including the nights (Deut. 17,3). If the Torah had only used the words חקות עולם, I might have concluded that this day is to be observed only during periods when the Jewish people are free and independent; when they are in exile, however, I could argue that there would be no occasion to especially mark that date, therefore the Torah had to add the word לדורותיכם to tell us that the Passover rites are to be observed not only when we are free and independent but even when we are in exile. It is a statute, חוקה, and does not depend on our circumstances.