Shavuot is determined by counting 50 days from the Shabbat following the Passover (Leviticus 23). Therefore, Shavuot falls in the third Biblical month. Shavuot is believed to be the anniversary of the time when Israel met God at Mt. Sinai (Exodus 19-24). It was at this time that the Law was given to Israel. God cut His covenant with Israel as the final phase of this event. Israel would be a holy people unto God from this day forth.
It was on the anniversary of this event, Shavuot, that God sent His Holy Spirit on the disciples in Jerusalem after the death and resurrection of Messiah Jesus. God wrote His Law in their hearts by the Holy Spirit. Believers were separated unto God for His special purpose from this time.
Thus Shavuot, commemorates the giving of covenant by God to His people. It celebrates the election of God; the separation of people for God’s own unique purposes. The election constitutes a “mystery” but we acknowledge that God has called and separated us for His purpose even though we may not understand why He chose us rather than someone else. Another “mystery” is that “…the gifts and the calling of God are irrevocable” (Romans 11:29). If we are a part of God’s people, we should celebrate His feast.