(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 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.