And this day shall be a memorial for you, and you shall celebrate it as a festival for Hashem; throughout your generations, as an everlasting statute you shall celebrate it...
The Mechilta explains why the possuk needs to tell us both לְדֹּרֹּתֵיכֶם – throughout your generations, and חֻקַת עוֹלָם – an everlasting statute. Both tell us that this Mitzvah is forever. The Mechilta answers that if we only had the possuk of לְדֹּרֹּתֵיכֶם, we would think that the Mitzvah only applies for two generations. The Torah added the words חֻקַּת עוֹלָם to teach us that this is forever, they need to keep this Mitzvah for all generations.
The Ohr Hachaim asks that the possuk still needs a pshat. If we need חֻקַת עוֹלָם, why did the Torah write לְדֹּרֹּתֵיכֶם?
The Ohr Hachaim answers based on a Gemara in Berachos. The possuk says לְמַעַן תִזְכֹּר אֶת יוֹם צֵאתְךָ מֵאֶרֶץ מִצְרַיִם כֹּל יְמֵי חַיֶיךָ - that you may remember the day when you came forth out of the land of Egypt all the days of your life. The Gemara says that the word יְמֵי חַיֶיךָ – the days of your life is referring to nowadays, but the word כֹּל is telling us that even in the future, when Moshiach is here, we will tell the story of when we left Mitzrayim. Another opinion in the Mishna is that we need to retell the story of leaving Mitzrayim during the daytime and at night. According to this opinion, how do we know that we must remember the Exodus from Mitzrayim when we are still in exile? When we are living a happy life, we are supposed to remember the happy days
The answer is in this possuk. Although one may think that we only have to thank Hashem for the redemption when we are in a situation of redemption, with freedom and good fortune, we need to remember this Mitzvah even during the Galus. That is the lesson from the words חֻקַת עוֹלָם. The Mitzvah applies forever, as a Divine decree.
However, once we know this Halacha, it is impossible that לְדֹּרֹּתֵיכֶם is only applicable to two generations. The Galus is many generations later, meaning that this possuk must be for eternal generations.
If the Torah did not write לְדֹּרֹּתֵיכֶם, we would mistakenly think that חֻקַת עוֹלָם does not mean even during the times of Galus. We would say that it means only during the time of redemption.
