Since the Church was not in existence until Pentecost, the Last Supper could not have been a Mass.
The Eucharist was instituted, yes. But a Mass was not said. Recall that the Mass is the re-presentation of the Sacrifice of Calvary.
That the ancient, complete and in every way perfect faith and teaching regarding the great mystery of the Eucharist in the Catholic Church may be retained, and with the removal of errors and heresies may be preserved in its purity, the holy, ecumenical and general Council of Trent, lawfully assembled in the Holy Ghost, the same legates of the Apostolic See presiding, instructed by the light of the Holy Ghost, teaches, declares and orders to be preached to the faithful the following concerning it, since it is the true and only sacrifice.
CHAPTER I
THE INSTITUTION OF THE MOST HOLY SACRIFICE OF THE MASS
Since under the former Testament, according to the testimony of the Apostle Paul, there was no perfection because of the weakness of the Levitical priesthood, there was need, God the Father of mercies so ordaining, that another priest should rise according to the order of Melchisedech,[1] our Lord Jesus Christ, who might perfect and lead to perfection as many as were to be sanctified. He, therefore, our God and Lord, though He was by His death about to offer Himself once upon the altar of the cross to God the Father that He might there accomplish an eternal redemption, nevertheless, that His priesthood might not come to an end with His death,[2] at the last supper, on the night He was betrayed, that He might leave to His beloved spouse the Church a visible sacrifice, such as the nature of man requires, whereby that bloody sacrifice once to be accomplished on the cross might be represented, the memory thereof remain even to the end of the world, and its salutary effects applied to the remission of those sins which we daily commit, declaring Himself constituted a priest forever according to the order of Melchisedech,[3] offered up to God the Father His own body and blood under the form of bread and wine, and under the forms of those same things gave to the Apostles, whom He then made priests of the New Testament, that they might partake, commanding them and their successors in the priesthood by these words to do likewise: Do this in commemoration of me,[4] as the Catholic Church has always understood and taught. For having celebrated the ancient Passover which the multitude of the children of Israel sacrificed in memory of their departure from Egypt,[5] He instituted a new Passover, namely, Himself, to be immolated under visible signs by the Church through the priests in memory of His own passage from this world to the Father, when by the shedding of His blood He redeemed and delivered us from the power of darkness and translated us into his kingdom.[6] And this is indeed that clean oblation which cannot be defiled by any unworthiness or malice on the part of those who offer it; which the Lord foretold by Malachias was to be great among the Gentiles,[7] and which the Apostle Paul has clearly indicated when he says, that they who are defiled by partaking of the table of devils cannot be partakers of the table of the Lord,[8] understanding by table in each case the altar. It is, finally, that [sacrifice] which was prefigured by various types of sacrifices during the period of nature and of the law,[9] which, namely, comprises all the good things signified by them, as being the consummation and perfection of them all.
Catholic Encyclopedia
Liturgy of the Mass
A. Name and Definition
The Mass is the complex of prayers and ceremonies that make up the service of the Eucharist in the Latin rites. As in the case of all liturgical terms the name is less old than the thing. From the time of the first preaching of the Christian Faith in the West, as everywhere, the Holy Eucharist was celebrated as Christ had instituted it at the Last Supper, according to His command, in memory of Him. But it was not till long afterwards that the late Latin name Missa, used at first in a vaguer sense, became the technical and almost exclusive name for this service.