Shavuot- The Day of Marriage

On Pesach (Passover), the people of Israel were freed from centuries of bondage and enslavement to Pharoah.  Led out of Egypt by Moses after the ten plagues, they crossed the Red (Reed) Sea into the desert.
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Fifty days later (Counting of the Omer) on Shavuot (Pentecost), the Torah was given on Mount Sinai.  Moses brought the bride (the nation of Israel)  to the bridegroom (God) at the Chuppah (Mount Sinai, represented by the canopy you see at most weddings).  The marriage contract (ketubbah) was the Torah.
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Shavuot is the fourth of the four spring feasts of God, the last of the spring feasts, and the fourth of seven annual feasts of God.  The four spring feasts were fulfilled the year Yeshua was crucified (Pesach/Passover), buried (Matzah/Unleavened Bread), rose again ( Bikkurim/ First Fruits) and sent the Holy Spirit (Shavuot/ Pentecost).
In what some refer to as the birthday of the church (Pentecost as described in Acts Chapter 2) the Holy Spirit (Ruach HaKodesh) descended on 120 apostles of Jesus Christ and other followers of Yeshua / Jesus.
The day the Torah was given (3000 people died) was the same day of the year centuries later that the Holy Spirit was given (3000 people were saved).
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Shavuot is also traditionally the day that King David both was born and died.