The main reason I say at Pentecost is because that is when the transformation of the Apostles took place and the promise of Jesus to tarry and wait as per John 14-16 with the promise of the Holy Spirit who in the future would be in them. In Acts 1 just prior to Jesus Ascension He told them the following and notice its still future.
Acts 1
On one occasion, while he was eating with them, he gave them this command: “Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. 5 For John baptized with water,
but in a few days you will be baptized with the Holy Spirit.”
6 Then they gathered around him and asked him, “Lord, are you at this time going to restore the kingdom to Israel?”
7 He said to them: “It is not for you to know the times or dates the Father has set by his own authority. 8
But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.”
9 After he said this, he was taken up before their very eyes, and a cloud hid him from their sight.
Remember Peter denied the Lord, feared his own life and scattered just like the other disciples. They remained in hiding until Pentecost. Peter was transformed that very day and without fear preached the Sermon in Acts 2 boldly before those same Jewish crowds he had feared. And when we read the following in the gospels we know the disciples did not even understand the gospel until Pentecost.
The disciples did not understand the death, burial and resurrection of Jesus which is the gospel.
Matthew 16:8-11
Aware of their discussion, Jesus asked, “You of little faith, why are you talking among yourselves about having no bread? 9
Do you still not understand? Don’t you remember the five loaves for the five thousand, and how many basketfuls you gathered? 10 Or the seven loaves for the four thousand, and how many basketfuls you gathered? 11
How is it you don’t understand that I was not talking to you about bread?
Mark 7:18
And he saith unto them
, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man,
it cannot defile him;
Mark 8:31-33
He then began to teach them that the Son of Man must suffer many things and be rejected by the elders, the chief priests and the teachers of the law, and that he must be killed and after three days rise again. 32 He spoke plainly about this,
and Peter took him aside and began to rebuke him.
33 But when Jesus turned and looked at his disciples, he rebuked Peter.
“Get behind me, Satan!” he said. “You do not have in mind the concerns of God, but merely human concerns.”
Luke 9:43-45
While everyone was marveling at all that Jesus did, he said to his disciples, 44 “Listen carefully to what I am about to tell you: The Son of Man is going to be delivered into the hands of men.” 45
But they did not understand what this meant.
It was hidden from them, so that they did not grasp it, and they were afraid to ask him about it.
Jesus predicts His death a 3rd time
Luke 18:31-34
Jesus took the Twelve aside and told them, “We are going up to Jerusalem, and everything that is written by the prophets about the Son of Man will be fulfilled. 32 He will be delivered over to the Gentiles. They will mock him, insult him and spit on him; 33 they will flog him and kill him. On the third day he will rise again.”
34
The disciples did not understand any of this. Its meaning was hidden from them, and they did not know what he was talking about.
Luke 24:25-26
He said to them,
“How foolish you are, and how slow to believe all that the prophets have spoken!
26 Did not the Messiah have to suffer these things and then enter his glory?
John 13
Jesus replied, “You do not realize now what I am doing,
but later you will understand…. I am telling you now before it happens, so that when it does happen you will believe that I am who I am.”