And you shall tell your son. I [would] understand from Rosh Chodesh [that one would have to observe Pesach. However] we learn [otherwise, since] it is stated, "on that day." If it is [written] "on that day," it could be from while it is still day [before the night of the fifteenth of Nissan. However] we learn [otherwise, since] it is stated, "for the sake of this;" when [this] matsa and maror are resting in front of you [meaning, on the night of the fifteenth].