Exodus, Leviticus, Numbers and Deuteronomy describe the biblical Feast of Weeks. These passages emphasize harvest, firstfruits, offerings and thanksgiving.
Exodus 19-20 tells the story of Israel at Mount Sinai and the giving of the Ten Commandments. The Torah itself does not state that these events are the reason for celebrating Shavuot. Jewish tradition later connected the festival with the giving of the Torah.
The book of Ruth is traditionally read at Shavuot. Its story takes place against the background of the grain harvest and includes scenes of harvesting in the fields.
Acts 2 provides the most important Christian connection. The Holy Spirit came upon the followers of Jesus on the day of Pentecost. Peter preached to the gathered crowd, and about three thousand people were baptized.
Acts 20:16 and 1 Corinthians 16:8 show that Pentecost continued to serve as a familiar point in the calendar during the ministry of Paul.