The “days of unleavened bread” were the 13th day through the 21st day of the 1st month on the Jewish calendar (Abib or Nisan 13th through 21st) (Matthew 26:17; Mark 14:12; Luke 22:7; Acts 12:3; 20:6). Under the Old Covenant during the Day of Passover (Nisan 14th) and the Feast of Passover (Nisan 15th through 21st), the Jews were forbidden to eat, or even to have, any leavened bread in their homes (Exodus12:8,17–20). That is why this feast is sometimes called the “Feast of Unleavened Bread.” In order to make sure that there was no leaven in their homes by sunset on the 13th of Nisan (Sunset on the 13th would signify the start of the 14th.), the Jews made sure that there was no leaven in their homes by sunset on the 12th of Nisan (Sunset on the 12th would signify the start of the 13th.). This gave them a one day “cushion” before the 14th in case they found any leaven in their homes on the 13th. Some people also killed the Passover lamb and prepared for the Passover meal during the day of the 13th. Then when the evening arrived (meaning it was now the 14th) they celebrated the Passover by eating the Passover meal. However, not everybody ate their Passover meal in the evening of the 14th. Some people waited until the daytime of the 14th to eat their Passover meal.
Note: It is important to note that in the King James Version (KJV) of the Bible, words that are not in the original Hebrew, Aramaic, or Greek are in italics. The italicized words are words that were added by the translators in order to make the text read smoothly in English. The translators did an outstanding job in translating the Hebrew, Aramaic, and Greek words into English. However, in some cases where they added the italicized words, the italicized words were added incorrectly. This is exactly why the translators added these words in italics. The translators wanted us to know that these words were not in the original languages that the Bible was written in. They wanted us to know that these italicized words were NOT inspired by God. They wanted us to know that these italicized words that they added may not be correct.
Such is the case in Matthew 26:17. The KJV reads:
“Now the first day of the feast of unleavened bread the disciples came to Jesus, saying unto him, Where wilt thou that we prepare for thee to eat the Passover?”
Notice that the word day and the words feast of are in italics. That means that these three words were not in the original Greek that the gospel of Matthew was written in. They were added by the translators. We know that the word feast should not be there because Mark 14:12 says:
“And the first day of unleavened bread, when they killed the Passover, his disciples said unto him, Where wilt thou that we go and prepare that thou mayest eat the Passover?”
and Luke 22:7–9 says:
“Then came the day of unleavened bread, when the Passover must be killed. And he sent Peter and John, saying, Go and prepare us the Passover, that we may eat. And they said unto him, Where wilt thou that we prepare?”
It is clear from these scriptures that Jesus and his disciples had not yet eaten the Passover meal. The Passover meal was always eaten on Nisan 14th. The Feast of Passover did not begin until the next day, Nisan 15th. So when the disciples were talking to Jesus about the preparation of the Passover meal, it was during the daytime of Nisan 13th. Neither the Passover nor the Feast of the Passover had started yet. Remember that the days of the Jewish calendar went from sunset to sunset—not midnight to midnight. So sometime during the daytime of Nisan 13th, Jesus' disciples prepared for them to eat the Passover meal. The Word of God then says in Matthew 26:20–21:
“Now when the evening was come, he sat down with the twelve. And as they did eat....”
and in Mark 14:17–18:
“And in the evening he cometh with the twelve. And as they sat and did eat....”
and in Luke 22:14:
“And when the hour was come, he sat down, and the twelve apostles with him.”
When the evening came, it became the 14th of Nisan. So Jesus and his disciples then ate the Passover meal.
Therefore, the disciples prepared for all of them to eat the Passover meal—including the killing of the Passover lamb—during the daytime of Nisan 13th. Therefore, Matthew 26:17 should have the italicized word feast replaced with the italicized word days and should read:
“Now the first day of the days of unleavened bread the disciples came to Jesus, saying unto him, Where wilt thou that we prepare for thee to eat the Passover?”
The disciples came to Jesus asking him where to prepare the Passover meal on the first day of the days of unleavened bread, Nisan 13th. They were not asking him where to prepare the meal on Nisan 15th, the first day of the feast—after the Day of Passover and the celebratory meal were over!
Some of the confusion arises because many Christians do not understand the Old Testament Jewish Calendar. They don't understand the difference between the Day of Passover when the Passover meal was eaten on the 14th of Nisan, and the Feast of Passover which ran for seven days from the 15th of Nisan through the 21st of Nisan. They also do not understand that the “days of unleavened bread” began the day before Passover, on Nisan 13th.