Forty days after Jesus was raised from the dead, he ascended into heaven. (Acts 1:3) About 10 days after that, on the Day of Pentecost, the Holy Spirit indwelled 120 of Jesus’ followers. (Acts 2:1-4) But, what was this “Day of Pentecost”?
“Pentecost” is a transliteration (sound-alike) of the Greek term that means fiftieth. In ancient Jewish tradition, Pentecost or Shavuot happened 50 days after the first Passover (as found in Exodus). It marks the day that God gave Moses the Ten Commandments on Mt. Sinai. Shavuot also marked the ending of the Passover season.
In the New Testament, Pentecost is only mentioned in Acts 2:1, Acts 20:16, and 1 Corinthians 16:8. The first occurrence marks the day (about 10 days after Jesus’ ascension) that the Holy Spirit indwells those first followers. The last two instances of “Pentecost” in the New Testament occur when Luke and Paul (respectively) mention that Paul desired to return to Jerusalem before Pentecost toward the end of his third missionary journey.
In the Old Testament, Shavuot is called the Festival of Weeks (which is the literal meaning of the word Shavuot) (Exodus 34:22 and Deuteronomy 16:10), the Festival of Reaping/Harvest (Exodus 23:16), and the Day of First Fruits (Numbers 28:26). The other ancient Jewish writings, other terms are used to indicate Shavuot (Pentecost).
Thus, Shavuot (Pentecost) commemorates both the giving of the Ten Commandments and the annual grain harvest in Israel. (There are certainly possible connections there with the giving of the Holy Spirit, but that connection is not directly made in the New Testament. Any connection – while possible and perhaps probable – would be speculative on our part. Of course, speculation is fun at times.)
Five books of the Hebrew Bible (Tanak) are known as the Megilloth. These books are Lamentations, Ecclesiastes, Esther, Song of Songs, and Ruth. In Jewish tradition, each of those books was associated with a certain festival. Ruth (as you might expect) was associated with Shavuot (Pentecost).
It was during this time that 120 disciples of Jesus were gathered in the room where Jesus and his disciples had their last meal together before his crucifixion. At Jesus’ request, the Father then sent the Holy Spirit to indwell his children (creating a new remnant), and they immediately began testifying about him to everyone they came into contact with.