Also known as Pentecost or Feast of Weeks. It falls at the end of the spring harvest. Extra fine flower was made from the spring harvest of barley or wheat by passing the flour through seven sieves. This fine flower was used to bake the only leavened bread used in the temple sacrificial system. These offerings were brought to the temple and were called first fruits. Every day those harvesting would bring the first shievs of harvest to the priest in the temple and were offered before G-d as a sacrifice. They were to be eaten by the priesthood.
The Bible tells us that the Ruach HaKodesh (Holy Spirit) came upon us on Shavuot (Acts 2). We are also told of our sages that Shavuot is the day that G-d gave us the Torah and spoke the ten commandments to us aloud (Exodus 20).