(Exodus 12:14) "And this day shall be for you as a remembrance": The day which is a remembrance for you, you celebrate. But we have not yet heard which day it is (that is a remembrance for you). From (13:3) "And Moses said to the people: Remember this day when you went out of Egypt, etc." the meaning still hangs in the balance (i.e., On which day did they go out?) It is written (Numbers 33:3) "And they journeyed from Ramses in the first month on the fifteenth day of the first month. On the morrow of the Pesach (Passover) offering the children of Israel left with a high hand." When did the children of Israel eat the Pesach? On the night of the festival. And they did not leave until the day of the festival itself.