Exodus 12:18-20 says,
18 "In the first
month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even.
19 Seven days shall there be no leaven found in your houses: for whosoever eateth that which is leavened, even that soul shall be cut off from the congregation of Israel, whether he be a stranger, or born in the land.
20 Ye shall eat nothing leavened; in all your habitations shall ye eat unleavened bread."
Leviticus 23:5-8 says,
5 "In the fourteenth
day of the first month at even
is the LORD'S passover.
6 And on the fifteenth day of the same month
is the feast of unleavened bread unto the LORD: seven days ye must eat unleavened bread.
7 In the first day ye shall have an holy convocation: ye shall do no servile work therein.
8 But ye shall offer an offering made by fire unto the LORD seven days: in the seventh day
is an holy convocation: ye shall do no servile work
therein."
According to the above passages, there are:
#1. 7 days of eating unleavened bread (Leviticus 23:6).
#2. 1st Day is an holy convocation (High Sabbath) (Leviticus 23:7).
#3. 7th Day is an holy convocation (High Sabbath) (Leviticus 23:8).
Here is what this looks like if Jesus and His disciples celebrated the Passover on the beginning calendar day of the 14th after the sun went down on Tuesday evening (Note: Tuesday is the 13th and Tuesday sundown begins Wednesday the 14th).
Here is what it looks like if Jesus and His disciples celebrated the Passover the 14th at the end of the day (i.e. the end of the 14th) on a Tuesday evening (Note: Sundown on Tuesday then began Wednesday the 15th).
The Jews thought the 15th was a day later. It is why the Scriptures stress the following about the Jew's version of keeping the preparation day at the wrong time.
"There laid they Jesus therefore because of the
Jews'
preparation day; for the sepulchre was nigh at hand." (John 19:42).
The real day of preparation of the lambs was the night before with Jesus. It's why we read the following.
"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?" (Matthew 26:17).