When did the disciples receive the Holy Spirit? Was it when the Spirit came upon them in the form of tongues as of fire (Acts 2)? Or when the Lord breathed on them and said to them, “Receive the Holy Spirit” (Jn. 20)?
They received the Holy Spirit for permanent dwelling on the day of Pentecost.
At that time the Lord’s promise was fulfilled that they would be, “Endued with power from on high” (Lk. 24:49), and also the promise, “If I do not go away, the Helper will not come to you; but if I depart, I will send Him to you” (Jn 16:7). This text shows that they were to receive the Holy Spirit after the Lord’s ascension to heaven, which happened on the day of Pentecost (Acts 2: 2-4).
But when the Lord breathed on them it was to give them the sacrament of the Holy Orders (Priesthood).
It is stated, “He breathed on them and said to them “Receive the Holy Spirit. If you forgive the sins of any, they are forgiven them; if you retain the sins of any, they are retained” (Jn 20:22, 23).
It means that He gave them by the Holy Spirit the authority to forgive sins, or rather He gave them the Spirit by whom they can forgive sins, thus forgiveness comes from God.
This breath that gave the Holy Spirit is confined to them, not for all believers.
It is given to those who were to perform the work of priesthood from among the apostles, disciples and successors, whereas the coming of the Holy Spirit on the day of Pentecost was for all and the apostles gave this gift to people by the laying on of hands (Acts 8:17), then by the Holy anointment (1 Jn. 2:20, 27) which is now given in the sacrament of Holy Chrism (Myron) to all believers.
Hence, the apostles received priesthood when the Lord breathed on them.
Then they took over this priesthood on the day of Pentecost when they baptized people.
The Lord knew that they were in need of Holy priesthood in order that they might baptize the new members of the church, loose and bind and practice all other sacraments. Therefore, He gave them the Holy Spirit – who was to give them priesthood – before giving them the Holy Spirit to dwell permanently in them as necessary for their ministry and lives as well.