C. F. Keil and Franz Delitzsch on Exodus 12:42

Exo 12:42

This day therefore was שׁמּרים ליל, “a preservation-night of the Lord, to bring them out of the land of Egypt.” The apax legomenon שׁמּרים does not mean “celebration, from שׁמר to observe, to honour” (Knobel), but “preservation,” from שׁמר to keep, to preserve; and ליהוה is the same as in Exo 12:27. “This same night is (consecrated) to the Lord as a preservation for all children of Israel in their families.” Because Jehovah had preserved the children of Israel that night from the destroyer, it was to be holy to them, i.e., to be kept by them in all future ages to the glory of the Lord, as a preservation.