Today, in many Christian churches, Pentecost Sunday is celebrated to recognize the gift of the Holy Spirit, realizing that God’s very life, breath and energy live in believers. During this service, John 20:19-23 may be the core of the message about our risen Savior supernaturally appearing to the fear-laden disciples. Their fear gave way to joy when the Lord showed them His hands and side. He assured them peace and repeated the command given in Matthew 28:19-20, saying, “As the Father has sent me, I am sending you.” Then He breathed on them, and they received the Holy Spirit (John 20:21-23).

The celebration of Pentecost Sunday reminds us of the reality that we all have the unifying Spirit that was poured out upon the first-century church in Acts 2:1-4. It is a reminder that we are co-heirs with Christ, to suffer with Him that we may also be glorified with Him; that the manifestation of the Spirit is given for the common good (1 Corinthians 12:7); that we are all baptized by one Spirit into one body (1 Corinthians 12:13); and that the Spirit which raised Jesus from the dead lives inside believers (Romans 8:9-11). This gift of the Holy Spirit that was promised and given to all believers on the first Pentecost is promised for you and your children and for all who are far off whom the Lord our God will call (Acts 2:39).

Source: https://www.gotquestions.org/Pentecost-Sunday.html