I want to summarize for you the reasons why that meal was, in fact, the Passover. I put the word "last" in quotation marks because it is inaccurate. For forty days after His Resurrection Jesus taught and ate meals with the disciples. I want to acknowledge the highly respected New Testament scholar D.A. Carson as my primary source for the following.
The "Synoptic" Gospels (Matthew, Mark and Luke) clearly speak of that meal as the Passover. Several statements in the Gospel of John seem to indicate that it was "before" the Passover meal. John as been misunderstood and misinterpreted. Here is why.
"Before the Passover Feast" (Jn. 13:1) refers to the act of washing the disciples feet. The KJV "supper being ended" was a mistranslation. The meal had "been served" but not yet eaten ("during supper").
The words in Jn. 18:28 :to eat the Passover" refer to the entire Feast of the Passover including the days of Unleavened Bread.
Considerable evidence exists to indicate that the words "day of Preparation" in Jn. 19:14 were a technical term for the Friday of Passover week. The Sabbath (Saturday) of Passover week was a "special" and "high" Sabbath. Thus the NIV has "the day of Preparation of Passover Week".
There remains, therefore, little if any reason to see any real discrepancy between the Synoptics and the Gospel of John. Jesus ate the Passover meal with his disciples.
The reader is referred to the fuller discussion of this in the Expositor's Bible Commentary, Vol. 8, pages 528-532. D.A. Carson was commissioned to write the commentary on Matthew (600 pages!) for this excellent series. He dealt with all the major problems relating to the four Gospels, including this one on the "Last" Supper. I disagree strongly with Carson on his interpretation of passages relating to the Tribulation and the Second Coming but in
this matter of the "Last" Supper being the Passover meal and in nearly all other matters I have the highest regard for his scholarship.