I don't know why you keep on asking me the same thing when the answer is already there.
Your idea is easily shown to be in error.
First let us look at this translation from the KJV in a little more detail:
"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?... Now when the even was come, he sat down with the twelve. And as they did eat, he said, Verily I say unto you, that one of you shall betray me. And they were exceeding sorrowful, and began every one of them to say unto him, Lord, is it I? And he answered and said, He that dippeth his hand with me in the dish, the same shall betray me" (Mt.26:17-18,20-23).
Now let us look at the same exact event described in the book of Mark:
"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 in the evening he cometh with the twelve....And as they sat and did eat, Jesus said, Verily I say unto you, One of you which eateth with me shall betray me. And they began to be sorrowful, and to say unto him one by one, Is it I? and another said, Is it I? And he answered and said unto them, It is one of the twelve, that dippeth with me in the dish" (Mk.14:12,17-20).
It is obvious to anyone with an open mind that the the events described in Matthew which I quoted are the same exact events described in Mark which I quoted. And these words from Mark prove that the day spoken of is not the first day of the feast of unleavened bread, as you imagine:
"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" (Mk.14:12).
The Passover lamb was killed before the 14th was over, not the 15th, the first day of the feast of unleavened bread:
"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" (Lev.23:6).
That contradicts your idea that Mark 14:12 and Matthew 26:17 are describing events of the first day of the feast of unleavened bread.
The observance of the Passover Seder was on the 14th so the Passover lamb must be killed by that day so it can be eaten:
"And in the fourteenth day of the first month is the Passover of the LORD" (Num.28:16).
Therefore, it is impossible that you are right when you assert that the words "first day of the feast of unleavened bread" is the correct translation in this verse:
"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?" (Mt.26:17; KJV).
Since this is the first time that I have made this argument it is impossible that you have answered it already. So if you want to defend your idea then you must address what I said on this post.