Because it was a custom to refer to both Passover and unleavened bread as "the feast of unleavened bread'.
https://www.sabbath.org/index.cfm/fuseaction/Library.CGGWeekly/ID/741/Is-Passover-on-First-Day-Unleavened-Bread-Part-One.htm#:~:text=Another critical point is that, despite Passover and,Abib 14-21 (Passover plus Unleavened Bread; Luke 22:1).
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Encyclopaedia Judaica confirms
:
- "The feast of Passover consists of two parts: The Passover ceremony and the Feast of Unleavened Bread. Originally, both parts existed separately; but at the beginning of the [Babylonian] exile they were combined" (vol. 13, p. 169)."
"At the time of
Jesus Christ, this mixture was on full display. Philo of Alexandria, in
De Vita Mosis, notes that in the early first century, the Passover was not strictly a Temple-kept event, but one in which people also killed their own lambs without help from the priests. In his
Wars of the Jews, Flavius Josephus records that in 4 BC over 250,000 lambs were sacrificed for Passover. However, given the limited space of the Temple environs and the fact that
Jewish tradition (not the Word of God) held that the lambs were to be slain within a two-hour time slot (from the ninth to the eleventh hour, or 3:00-5:00 pm), it is readily apparent that not all of those lambs could have been sacrificed at the Temple. In fact, Joachim Jeremias, in
Jerusalem in the Times of Jesus, calculates that the three courses of priests on duty could slay only 18,000 lambs during those two hours. Josephus records that the rest of the lambs—a far greater number—were slain by individuals at their own homes.
"Another critical point is that, despite Passover and the Feast of Unleavened Bread being distinct festivals, they were commonly grouped together and simply called "Passover." Thus, when
the gospel writers mention "Passover," it can sometimes refer to the Passover sacrifice itself (
Matthew 26:17;
Mark 14:12), the day when the sacrifice was made (
Mark 14:1), or the whole eight-day period of Abib 14-21 (Passover plus Unleavened Bread;
Luke 22:1).
"That the disciples inquired about making preparations—and later that night assumed Judas would be purchasing something "for the feast" (
John 13:29)—shows that the time in question could not have been the first day of the Feast of Unleavened Bread. Why? That day is a holy convocation on which no customary work is to be done (
Leviticus 23:7), if God's instructions are to remain unbroken."