Question 5.13: What happened to the observance on 14 Nisan as Passover?

                                  Answer:
   
   Actually, the Torah only refers to the afternoon of the 14th as
   Passover. This is the time during which the Passover offering was
   brought. It doesn't even imply that the day was named "Passover", just
   that it refers to that period as "during the Passover [offering]". The
   offering was actually eaten that night, on the fifteenth, the begining
   of the holiday the Torah calls Chag haMatzos (the holiday of matzahs).
   
   Why the name change? We call the holiday "Passover" to commemorate
   what G-d did for us. He passed over the Israelites' homes and saved
   them from Egypt. However, in writing the Torah, G-d stresses what man
   did and does. So in the text of the bible, the name is taken from the
   commandment of the day.