Parshanut zu Schemot 12:57

A festival to Hashem. This festival was not to commemorate their release from servitude but their recognition as a people by Hashem. That is why it is “an eternal statute,” to be observed even if they should subsequently become enslaved again, for instance in the time of Achashverosh. For the Divine recognition and the special providence that began at that time never cease (Meshech Chochmah).