We know that the Church considers the Last Supper (the first Mass ever) to be the event that instituted the priesthood. And of course, there can be no Mass and no Eucharist without a priest (a bishop is a kind of priest). So that makes sense.

On the other hand, the Apostles were not ordained at that point in time. It seems rather that their formal ordination was after the death and resurrection, when Jesus breathed the Holy Spirit on them:

[John]
{20:19} Then, when it was late on the same day, on the first of the Sabbaths, and the doors were closed where the disciples were gathered, for fear of the Jews, Jesus came and stood in their midst, and he said to them: “Peace to you.”
{20:20} And when he had said this, he showed them his hands and side. And the disciples were gladdened when they saw the Lord.
{20:21} Therefore, he said to them again: “Peace to you. As the Father has sent me, so I send you.”
{20:22} When he had said this, he breathed on them. And he said to them: “Receive the Holy Spirit.
{20:23} Those whose sins you shall forgive, they are forgiven them, and those whose sins you shall retain, they are retained.”

Forgiveness of sins requires the priesthood, ordination to the sacerdotal degree. So this must have been their ordination. (I don’t think they could have been ordained as deacons prior to this event.) And it seems that they were ordained by Christ directly as Bishops, as there was no subsequent event that would raise them from the priesthood to the episcopate.

So the institution of the priesthood was a two-step process (or perhaps three steps, including Pentecost). It began on Holy Thursday, but actual ordination occurred after the Resurrection, before the Ascension. At Pentecost, then, the Church received a type of Confirmation in the Holy Spirit, though of course the Spirit was already with the Church prior to Pentecost.

Thanks be to God, the most holy Trinity, for the holy Eucharist, which is Jesus under the appearances of bread and wine.

RLCJ