I understand your position fully and completely. But I'm not sure you understand mine. I hope you will read the following,
carefully and thoughtfully. I don't think of it as "my position" but rather The Bible's Position, as illustrated by those three anchor verses posted earlier that are at pains to say,
"Not on the feast", "Not on the feast", and "Now before the feast". Here goes:
The principle must not be infringed, that Jesus came to fulfill the Law. Any interpretation that breaks that principle is incorrect. Jesus is The Passover Lamb. Who wants to claim otherwise? When is the Passover to be killed? 15 Nisan? Incorrect. If the fulfillment of the Law, Jesus Christ, the true Passover Lamb is not being killed on 14 Nisan, then God is not in His Heaven. Who thinks God doesn't know the day and the hour of Passover? Think about that. How many interpretations deny, unintentionally, that God is incompetent? The answer to that question is scary.
How then do we interpret passages like Mark 14:12-16? What did "The first of Unleavened" mean in Jesus' day?
"Unleavened" had become the traditional label for "Passover + The Feast of Unleavened Bread." Since this was the greatest festival of the new year, it was for them like Christmas is for us. There was much preparation that had to be done in advance because the actual feast's eating restrictions required planning. Most of that preparation had to be completed by 13 Nisan. Because all the "suppers" of the festival itself were more strict, it became traditional to have one last normal meal/feast before Passover, almost like a Thanksgiving Day meal or an "anything goes" meal because we're in store for a week of Law restrictions. This "last hurrah" supper was called, "the preparation meal", partaken at the end of 13 Nisan, "Before the feast of Passover". This meal had become a huge tradition. HUGE as in big. Was this meal a part of the Law? No. Was it tradition? Yes, and it seems on the whole a rather harmless tradition. This tradition had over time been included as a feature of the festival, "The Festival of Unleavened".
Since it had become part of the festival in Jesus' day, what day is "The first of unleavened"? 13 Nisan. You will notice in Greek, the word "bread" is not there but is included in most English translations. In Greek, it's, "first of Unleavened", not "first of unleavened [bread]". The translators are trying to be "helpful" but are in fact making it harder for the layman to figure out that it's not the actual first day of the feast proper. (Thank you, translators! Not)
John to the rescue.
That's why John prefaces his description of what occurred at the meal in the upper room (and everything else) with "Now before the feast of Passover". God made sure we had an anchor. John is describing what happened during the epic "preparation meal". And now we fully understand what the passages in Matthew, Mark and Luke mean when they speak of "preparing the Passover" (on 13 Nisan, last and biggest day of preparation) and "The first of unleavened". They are talking about the 13 Nisan epic meal before the Festival Requirements take effect.
Thank God for John.
John prefaces everything in 13:1 with "Now before the feast of Passover...", you know, BEFORE THE FEAST OF PASSOVER and they eat this meal, BEFORE THE FEAST OF PASSOVER, the traditional preparation meal which in those days had become a prominent feature of the festival of "Unleavened", and had become part of the festival.
By tradition, the first of unleavened is the preparation meal of 13 Nisan, not the first as defined by Law. That's what Matthew, Mark and Luke mean by "first of unleavened" -- the first as defined by tradition, not the first as defined by the Law. John confirms this in 13:1 by defining his "first" by Law. So in John, the upper room meal occurs before Feast of Passover as defined in the Law. John describes what the others describe as "the first [event] of unleavened" as had become the tradition of the day, but makes it clear that all of this occurred before the feast of Passover. All of it, until Christ's death on the cross.
Thank you for taking the time to arrive at a full understanding of "my argument". If you still have questions, I'd be happy to attempt answers.