Pentecost is a Greek word meaning "the fiftieth day." The feast of Pentecost was celebrated 50 days after the Jewish Passover. (When counting, omit the Sabbaths.) The Jewish feast of Pentecost was the festival giving thanks for the first fruits of the corn harvest. Since Jesus' crucifixion happened on the day after Passover, that would mean that there are 49 days between the crucifixion and the coming of the Holy Spirit.
Acts 2:1 notes that on the Day of Pentecost, the Holy Spirit descended on Jesus' disciples. This event was called Pentecost by the early Christian Church. Pentecost is observed as the birth of the Christian Church.
"When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them." —Acts 2:1-4
Related resources for your congregation:
A Quiet Pentecost (Upper Room Books, 2013)
Pauses for Pentecost (Upper Room Books, 2018)
We Are the Church ... Let's Act Like It (Upper Room Books, 2019)