Although Passover is traditionally recognized as an eight-day holiday, technically speaking it refers to the first night of the sacrifice of the lamb of God and the first day of the Exodus from Egypt (i.e., Nisan 15). The following week the people ate unleavened bread as they journeyed out Egypt and finally crossed the sea on the eighth day. Of particular importance, however, is the Passover night, which the Lord commanded Israel to observe forever as a vigil in memory of their great deliverance from Egypt.
“At the end of 430 years, on that very day (i.e., Nisan 15), all the hosts of the LORD went out from the land of Egypt” (Exod. 12:41). Immediately after stating this, the Torah refers to Passover as leil shimurim (ליל שמורים), a “night to be guarded” (from the verb shamar, which means “to watch” or “to guard”). “It is a night that is guarded (leil shimurim) to the LORD for bringing them [Israel] out of Egypt; this night remains a night to be guarded (שִׁמֻּרִים) by the people of Israel throughout their generations” (Exod. 12:42). Since “this night” – ha-lailah hazeh (הלילה הזה) – was guarded by God from the beginning to be the time of redemption, Israel must therefore “guard this night” (i.e., keep a vigil) by means of the Passover Seder (Exod. 13:10; Deut. 16:1), recalling and celebrating God’s faithfulness and redemptive love.