Pentecost. What is it?
Steve Bond gives this helpful explanation in the Holman Illustrated Bible Dictionary.  Bond gives the historical context and the New Testament fulfillment.
One of three major Jewish feasts also called the Feast of Weeks. The name “Pentecost” is derived from the Greek word meaning “fifty.” Pentecost occurs in the month of Sivan (May/June), 50 days after Passover, and celebrates the end of the grain harvest. The Pentecost that followed Jesus’ death and resurrection was the occasion on which the Holy Spirit was given to believers in Jerusalem.
Believers were together celebrating Pentecost when suddenly unusual sights and sounds signaled an event that would have far-reaching implications. They heard the sound of “a violent rushing wind” (Acts 2:2 HCSB). They saw tongues of fire resting on believers, and each believer was able to communicate in languages they had never before spoken. They were able to speak with Jewish pilgrims of many languages from all over the Mediterranean world.
These phenomena got the attention of the Jewish faithful gathered in Jerusalem. Many thought the disciples were intoxicated. But Simon Peter got up and pointed out that it was too early in the day for people to be drunk. Peter then took this unusual opportunity to proclaim that this event was the fulfillment of Joel’s prophecy that God would pour out His Spirit on all people. Peter linked the gift of the Spirit with the life, death, resurrection, and ascension of Jesus.
Peter’s message found its way into the hearts of over 3,000 who responded by repenting and being baptized in the name of Jesus and receiving the gift of the Holy Spirit.
What does this mean for us?
Pentecost is a time of celebration for Christians today! It is a reminder that God has fulfilled His promise. It also reminds us that we have the Holy Spirit living within us as well. That same Spirit that came in power on the day of Pentecost, is the same Spirit who raised Jesus from the grave, is the same Spirit that lives in the believers of Jesus.
The Holy Spirit can move powerfully in our lives to spread the gospel to those around us, just like He moved powerfully in the lives of the disciples. Are you connected to the Spirit (John 15) in a way that will allow Him to speak powerfully through you?
 Steve Bond, “Pentecost,” ed. Chad Brand et al., Holman Illustrated Bible Dictionary (Nashville, TN: Holman Bible Publishers, 2003), 1272–1273.