The ordinance of the Passover is to slaughter the sacrifice at twilight of the fourteenth day of Nissan as a remembrance of the Passover in Egypt. The moon becomes complete on the fifteenth day, and the sacrifice should be slaughtered at twilight of the fourteenth day because that is the time that Judgment hangs over the world. The main part of the commandment is to gain pleasure from the scent that spreads from the roasting over the fire. Only one who is circumcised can eat of it. At the time when God came to Egypt and saw the blood marked on the entrances of the houses of Yisrael, the people used hyssop to spread the blood, since hyssop removes evil spirits and any aspect of a bad odor because it arouses the supernal Redemption of Yisrael. In the time to come, God will slaughter the Evil Inclination. Because God slew all the firstborn of the Egyptians, He obligated all the firstborn of Yisrael to redeem themselves, and He guarded them against everything. Scripture says, about the sacrifice, "Neither shall you break a bone of it," because the bones were the deities of the Other Side, and the children of Yisrael threw them out in order to express their contempt for the Egyptian deities.