What happened to the observance on 14 Nisan as Passover?
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). There is partial observation on the afternoon of 14 Nissan, as the afternoon is the preferable time to make the Matzat Mitzva.
