Leviticus 23 outlines several “feasts” (a.k.a. festivals or holidays) that God expected the people of Israel to observe. Three of these feasts required a pilgrimage to Jerusalem to present a “first-fruit” offering to the Lord (see Exodus 23:14-17; 34:22-23 & Deuteronomy 16:16), namely the Feast of Unleavened Bread (a.k.a. Passover), the Feast of Weeks (a.k.a. Pentecost), and the Feast of Tabernacles (a.k.a. the Feast of Booths/Tents). Jews made these three pilgrimages annually until the Temple in Jerusalem was destroyed in ad 70.
The Feast of Weeks was prescribed to take place seven weeks (seven sevens) after Passover, which, counting Passover, puts it on the 50th day. The Jews during New Testament times simply referred to the Feast of Weeks as the “Fiftieth,” or in the Greek, πεντηκοστή (pentekoste), which in English is Pentecost. At Pentecost, the Jews celebrated God presenting the Torah (the “Law,” i.e. the first five books of the Old Testament) to the people of Israel through Moses at Mount Sinai.
Acts 2 gives an account of the first Pentecost (Feast of Weeks) celebration after the death and resurrection of Jesus. As one of the three pilgrimage festivals, Jews from all lands were gathered in Jerusalem. On this day, the Holy Spirit came in a special way and enabled the apostles to present the Gospel to all the people gathered in their own languages.
Pentecost has been observed in the Church since the third century, and has at times been referred to as Whitsunday (from the white garments worn by those being baptized on Pentecost).
Peter gives a powerful sermon at Pentecost, less than two months after the death and resurrection of Jesus. Peter connects the events of that day with a prophecy from Joel 2, concluding the quote with, “Everyone who calls on the name of the Lord will be saved.” Peter attests to the resurrection of Jesus in Acts 2:32, and the culmination of his message is recorded in verses 38-39, “Repent and every one of you be baptized in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. For the promise is for you and your children and for all who are far off, everyone whom the Lord our God calls to Himself.”