Shavuot also known as the Festival of Weeks or the Day of Pentecost is first referenced in the Book of Exodus 34:22. Its timing in the month of Sivan (6) does not allow for any type of Eclipse to occur during or very close to this festival.
9 Seven weeks shall you number to you: begin to number the seven weeks from such time as you begin to put the sickle to the corn. 10 And you shall keep the feast of weeks to the LORD your God with a tribute of a freewill offering of your hand, which you shall give to the LORD your God, according as the LORD your God has blessed you:
Prior to this festival is the Counting of the Omer, where forty-nine days are counted between Passover and Shavuot. The Omer is a sacrifice containing an omer measure of barley; this was offered in the temple until the day before the wheat offering was brought into the temple on Shavuot, which was the Fiftieth Day. This is where the name Pentecost was established.
This period is also associated with the time span between the Passover and the giving of the Torah by God on Mount Sinai. This again is a foreshadow of the things that God had planned to be revealed in time to come. After the crucifixion of the Lord Jesus Christ at Passover, he showed himself to be alive to his disciples for a period of forty days:
3 To whom also He showed Himself alive after His passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God: 4 And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, says He, you have heard of Me. 5 For John truly baptized with water; but you shall be baptized with the Holy Ghost not many days from now.
Later, on the day of Pentecost or Shavuot, the promise of the Holy Spirit was fulfilled; hence the pre-ordained divine timing of God’s plans can be observed. Firstly at Passover the Lamb of God was slain and then at Pentecost the promise of the Comforter or the Holy Spirit was then given:
And when the day of Pentecost was fully come, they were all with one accord in one place. 2 And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. 3 And there appeared to them cloven tongues like as of fire, and it sat upon each of them. 4 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
This festival is normally observed around May to June.