Following Jesus' ascension, the disciples did just as Jesus had commanded them -- they remained together in Jerusalem and waited on the giving of the Holy Spirit before they continued with what is known as the Great Commission. (Jesus said, as recorded in Acts 1:8 -- "But you shall receive power when the Holy Spirit has come upon you; and you shall be witnesses to Me in Jerusalem, and in all Judea and Samaria, and to the end of the earth." NKJV) As we read in Acts 2:1-4 (shown above), the Holy Spirit was given on the Day of Pentecost, empowering the disciples to speak in other languages, resulting in the message of the Gospel being shared simultaneously with thousands of people!
But why did this happen on the Day of Pentecost? Certainly, there were a lot of people in Jerusalem -- this was a celebration day that would be accompanied by a lot of activity. So, from a strategic sense, there was good reason for this to happen on this day. However, could there be more? I think so...in fact, from my initial study of Scripture, it appears to be linked to the Old Testament in a way that completely glorifies God!
First, it's important to understand the significance of the Day of Pentecost. The word "Pentecost" refers to "fifty days". In the Old Testament, the Day of Pentecost is directly linked to the Jewish festival "Shavuot", in the Bible known as the Festival (or Feast) of Weeks. It is a celebration that commemorates when God gave Moses the 10 Commandments on Mount Sinai -- an event that occurred fifty days after the Passover, when the Jewish people were freed from bondage and enslavement to the Egyptians. (From Wikipedia -- "On Passover, the Jewish people were freed from their enslavement to Pharaoh; on Shavuot they were given the Torah and became a nation committed to serving God.")
Interestingly, the Festival of Weeks is also referred to in Scripture as Day of First Fruits, a celebration of the harvest that God gave His people, and is linked to the Feast of Harvest (see Exodus 23:16 ) as well. God's people would celebrate how God had taken care of them and would offer back to God the first-fruits of their harvest as a way of honoring God.
Here's where it gets so incredible -- look at the similarities and the significance of the two events, from the Old Testament and the New Testament:
1) In the Old Testament, the Passover celebrated when the Jews were freed from enslavement and Pentecost celebrated when they became a nation committed to God. In the New Testament, the crucifixion of Christ during Passover week is something we look back to as a time when humanity was provided freedom from being enslaved to sin...and at Pentecost, through the giving of the Holy Spirit, it's a celebration of entering a new relationship with God.
2) In the Old Testament, as you read the accounts, you find that -- when God gave the law (the Torah) to Moses on Mount Sinai, He wrote the 10 Commandments Himself on the tablets of stone. (see Deuteronomy 9:10 ) God Himself carved the 10 Commandments into the stone to signify the covenant relationship He had with His people. In the New Testament, the image of the Holy Spirit being given to the disciples mimics the finger of God -- in this case, it was as tongues of fire, signifying the beginning of a new covenant relationship God was initiating with all of humanity, to all those who would trust in His Son, Jesus Christ. In both cases, God "sealed the deal" symbolically through a direct intervention.
3) Of course, with Pentecost being directly tied to the Feast of Harvest, there is a direct correlation to the harvest the disciples experienced following the giving of the Holy Spirit. Acts 2:41 tells us the following: "Then those who gladly received his word were baptized; and that day about three thousand souls were added to them." (NKJV) And, just as the Jewish people would make a sacrifice of first-fruits on this day, we see the early church members also making a sacrifice of their own -- Acts 2:44-45 says, "Now all who believed were together, and had all things in common, and sold their possessions and goods, and divided them among all, as anyone had need."
These are just some parallels that I felt God impressing upon my heart as I was preparing for a Bible study in 2009. While not meant to be exhaustive in any way, for me it just serves as a reminder of how intentional God is about everything He does... He is truly amazing!