The following quote from Christianity.com is helpful as we reflect on the origins of Pentecost Sunday. There are many connections with the Old Testament and the details surrounding the birth, death, resurrection and ascension of Jesus.
In the Old Testament, you will discover that Pentecost was one of the Jewish feast days. Only they didn’t call it Pentecost. That’s the Greek name. The Jews called it the Feast of Harvest or the Feast of Weeks. It is mentioned five places in the first five books — in Exodus 23 & 24, Leviticus 16, Numbers 28 and Deuteronomy 16.
Pentecost was the celebration of the beginning of the early weeks of harvest. In Palestine, there were two harvests each year. The early harvest came during the months of May and June; the final harvest came in the Fall. Pentecost was the celebration of the beginning of the early wheat harvest, which meant that Pentecost always fell sometime during the middle of the month of May or sometimes in early June.
And here Peter stands up and says, “And let me tell you about the Messiah, who is the eternal protection for all of humanity. Let me tell you who he is.” And so Pentecost becomes this marker in history to really what many people would say, “And that’s the moment that church is born.”
That is when thousands come into the faith. And it goes from this little sect of believers who followed a Jewish rabbi from Nazareth who died and rose again, and suddenly the church breaks forth into the culture. Suddenly it is that unstoppable force that no one can really deny any longer. Pentecost has taken on a new significance for us. It, at one point, is just a historical memory. Now it is the living reality of the moment that the spirit of God seemingly burst forth.
From Christianity.com
Jesus said, “But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you.” John 14:26, ESV.