The day called “Pentecost” is named
after the Greek word pentekostos, which means “fiftieth.” It was the
only Old Testament festival determined by counting. On the day after the
Sabbath after Passover, the ancient Israelites selected a sheaf of the first
grain that had been harvested in the spring. This grain became an offering, and
the priest waved it “before the Lord” (Leviticus 23:11-12). Pentecost was
observed in ancient Israel on the 50th day after this (verse 15).
After his resurrection, Jesus appeared to his disciples and instructed them for 40 days, after which he ascended to heaven. While with them, he said: "Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptized with water, but in a few days you will be baptized with the Holy Spirit" (Acts 1:4-5). That first baptism of the Spirit would be the birthday of the church.
We can read what happened, but we simply cannot experience what Jesus' disciples felt after Jesus was resurrected. They had already seen more miracles than most people would ever believe. They had heard the message for three years and still did not understand it, and yet they continued to follow Jesus. His boldness, his confidence, his sense of destiny made him strangely attractive. But the crucifixion was a devastating blow. All their hopes were smashed. Their excitement turned to fear—they locked the doors and made plans to go home, back to the jobs they used to have.
Jesus promised his disciples: "I am going to send you what my Father has promised; but stay in the city until you have been clothed with power from on high" (Luke 24:49). Luke repeats the promise in his introduction to Acts: "Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptized with water, but in a few days you will be baptized with the Holy Spirit" (Acts 1:4-5).