The possuk is saying that Hashem took us out of mitzrayim because of “this”, i.e. to do this. We know that Hashem took us out of Mitzrayim to do all the mitzvos of Pesach, including matzah and maror, not just the korban pesach. So therefore it must be referring to night, when we keep all these mitzvos, and not the day, when we are only doing one of them.
Menu