Originally, this was the second feast in rank for the Jews, the celebration of thanksgiving for the harvest and the ending of Passover time. Later it was also to become a celebration of the giving of the law to Moses at Sinai.
In Christian recognition, Pentecost is the feast celebrated fifty days after Easter or ten days after the Feast of the Ascension. It commemorates the descent of the Holy Spirit on the Apostles. It marks the beginning of the active apostolic work and is hailed as the birthday of the Church, for it was through the coming of the Holy Spirit that the Church began to form members of the new kingdom.
As St. Peter spoke on the first Pentecost of the new covenant: “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the holy Spirit” (Acts 2:38).
It is the Spirit who makes effective the New Law and by coming to each one, enables us to profit by and fulfill the acts necessary for salvation. (8:28)