Holy Gospel of Jesus Christ according to Saint John 20:19-23.
On the evening of that first day of the week,
when the doors were locked, where the disciples were,
for fear of the Jews,
Jesus came and stood in their midst
and said to them, "Peace be with you."
When he had said this, he showed them his hands and his side.
The disciples rejoiced when they saw the Lord.
Jesus said to them again, "Peace be with you.
As the Father has sent me, so I send you."
And when he had said this, he breathed on them and said to them,
"Receive the Holy Spirit.
Whose sins you forgive are forgiven them,
and whose sins you retain are retained."
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You can say that early Christians have a preoccupation with the first day of the week. The Resurrection happened on such a day. Now, even the appearance of the risen Jesus also fell on such a day. The first day of the week, of course, is Sunday. St. John the Evangelist, takes things a little bit farther, and to stress an important truth of faith, lumped all the events that we know of Easter, together in one day: the Resurrection, the appearances, and the giving of the Holy Spirit.
For St. John, the outpouring of the Holy Spirit, signified in Jesus' breathing on the disciples, is the fruit of His Passion, Death, and Resurrection. God acts as One. The Son and the Spirit working in unity with the Father for our Salvation. The breathing forth remembers the events of Creation, when the Spirit of God hovered over the primordial waters and when Yahweh breathed life into man. The great work of Salvation that begun in Creation is accomplished today. God, through the Son, did not only give us His flesh and blood, but now has given us the fullness of His Spirit.
Pentecost is the defining moment of Salvation when we are fully confirmed in the Trinity. We have not only become adopted sons and daughters of the Father, nor partakers of Jesus' Divinity in His Body and Blood, but now we have become Temples of the Spirit. Salvation is all about God taking us into Himself, uniting us into Himself, and loving us into Himself. The power, authority, and gifts that the Spirit showers upon us in the Church, is but natural consequence of our entry into God.
