Today we observe what we call "Passover" or "Pesach" beginning on the evening of the 15th of Nisan, and then for seven days in Israel and eight days elsewhere. But the Torah, at Lev. 23:5 and Exodus 12 describes "Passover" or "Pesach" as the day the Pesach offering was brought in the Temple -- the 14th of Nisan -- and Lev. 23:6-7 tells us that the Festival of Matzos begins on the following night (the 15th of Nisan) and continues for seven days. During the first night (15th of Nisan), the Torah commanded the Jewish people to eat the Pesach offering made the afternoon before (on the 14th of Nisan). Exodus 12:6-8. Also, during the first night of the Festival of Matzos we were to retell the story of our redemption from Egypt.
Since all of these things occurred on what the Torah calls the Festival of Matzos, why do we call the festival "Passover" instead of by its Biblical name?