In the Church’s annual liturgical cycle, Pentecost is “the last and great day.” It is the celebration by the Church of the coming of the Holy Spirit as the end—the achievement and fulfillment—of the entire history of salvation. For the same reason, however, it is also the celebration of the beginning: it is the “birthday” of the Church as the presence among us of the Holy Spirit, of the new life in Christ, of grace, knowledge, adoption to God and holiness.

https://oca.org/saints/lives/2019/06/16/45-holy-pentecost