It is a night to be much observed unto the Lord for bringing them out from the land of Egypt: this is that night of the Lord to be observed of all the children of Israel in their generations. (Exodus 12:42)
That is, the night of the eating of the Passover meal, concerning which directions had already been given (see vs. 6-11, 14). Its mingled horror and rejoicing could never be erased from the memory of a people who owed their birth as a nation to that memorable night.