Because the LORD kept a vigil that night to bring them out of the land of Egypt, this same night is to be a vigil to the LORD, to be observed by all the Israelites for the generations to come.
Jewish Publication Society of America Version (JPS1917) - Exodus 12:42 >It was a night of watching unto the LORD for bringing them out from the land of Egypt; this same night is a night of watching unto the LORD for all the children of Israel throughout their generations.