Hidden in the Greek of Matthew 26:17; Mark 14:1, 12; and Luke 22:7 is a reference to Passover as "the first of the unleaveneds." This is because unleavened bread is indeed used on the 14th as part of the Passover service. A comparison with the Old Testament, however, discloses this to be only the popular usage of some during New Testament times. In the Old Testament, something akin to this is found in Deuteronomy 16, where the first day of Unleavened Bread is called "Passover," while the context clearly describes the first day of Unleavened Bread. People popularly used Passover and Unleavened Bread interchangeably, and the Bible notes this practice, though "Passover" was the term most generally used for the whole period.
Doing things like this is not uncommon. Today, we commonly refer to the Feast of Tabernacles and the Last Great Day as either the "Feast" or "Tabernacles," even though we clearly understand that the Feast of Tabernacles and Last Great Day are separate festivals. So it was with Passover in the time of Christ and the apostles. Neither our use of "Tabernacles" nor the Jews use of "Passover" alters the authority of God's intent in the Scriptures.
John W. Ritenbaugh
Countdown to Pentecost 2001