About fifty days after leaving Egypt, God’s people had a Mount Sinai experience. God had appeared to Moses in a burning bush on Mount Sinai as Moses was shepherding sheep. God called Moses to go back to Egypt and He would use him to free the Hebrew people from the Egyptians. After a short debate, God gave Moses a sign. Exodus 3: 12 God answered, “I will be with you. And this is your sign that I am the one who has sent you: When you have brought the people out of Egypt, you will worship God at this very mountain.” 
When God gave His people the Feast of Weeks, it was instituted as a second first fruit celebration of the wheat harvest. Yet, the timing of the festival, fifty days after Passover was also the timing of the arrival of the people at Mount Sinai and as a result, the giving of the law at Sinai was also celebrated during the Feast of Weeks or Pentecost.
The First Mount Sinai Experience
God wanted to meet with His people at Sinai and gave instructions on how they should do this. God appeared in smoke and fire which frightened the people. They asked that Moses to go meet with God on their behalf. So, Moses went up the mountain to meet with God. But the people thought that Moses was taking too long. Perhaps this God of fire and smoke may have killed Moses.
Exodus 32:1When the people saw how long it was taking Moses to come back down the mountain, they gathered around Aaron. “Come on,” they said, “make us some gods who can lead us. We don’t know what happened to this fellow Moses, who brought us here from the land of Egypt.” So the people built a golden calf and began to worship it instead of the true God.
God was angry and sent Moses back down the mountain. As a result of the judgment of God, three thousand Hebrew people were killed because of their sin. Moses then interceded for the people and the Lord forgave them.
A Second Mount Sinai Experience.
The Apostle Paul trained Luke who wrote the book of Acts. Paul would have trained Luke in a Jewish world view. Just as there was smoke and fire at the first Sinai experience, there was tongues of fire and wind at Pentecost. At the first Sinai experience the people’s sin caused three thousand to die. But after the Holy Spirit enabled the disciples to tell the good news in multiple languages, three thousand people were added to the church.
Acts 2:40 Then Peter continued preaching for a long time, strongly urging all his listeners, “Save yourselves from this crooked generation!” 41 Those who believed what Peter said were baptized and added to the church that day—about 3,000 in all.
The law that was given in the original Mount Sinai experience reveals sins that condemns us, but the Holy Spirit who came on Pentecost reveals a Savior who saves us. So, the fulfilment of the Feast of Weeks was revealed as the Holy Spirit was given.
To hear more about a Mount Sinai experience, listen here: