Pentecost coincides with the Feast of Weeks which is counted as 7 weeks following the second day of Passover.   

And the feast of harvest, the firstfruits of thy labours, which thou hast sown in the field: and the feast of ingathering, which is in the end of the year, when thou hast gathered in thy labours out of the field. Exo 23:16

And thou shalt observe the feast of weeks, of the firstfruits of wheat harvest, and the feast of ingathering at the year`s end. Exo 34:22

Also in the day of the firstfruits, when ye bring a new meat offering unto the Lord, after your weeks be out, ye shall have an holy convocation; ye shall do no servile work: Num 28:26

And thou shalt keep the feast of weeks unto the Lord thy the Lord with a tribute of a freewill offering of thine hand, which thou shalt give unto the Lord thy the Lord, according as the Lord thy the Lord hath blessed thee: Deu 16:10

Ye shall bring out of your habitations two wave loaves of two tenth deals: they shall be of fine flour; they shall be baken with leaven; they are the firstfruits unto the Lord. Lev 23:17

Early in the month of Sivan Moses received the 10 Commandments , came down from Mt, Sinai, found the children of Israel worshipping a golden calf and 3000 people died.   

On the day of Pentecost (final day of Feast of Weeks) when the Disciples were together in the upper room, they were are filled with the Holy Spirit , Peter preached to the public and 3000 people received Jesus Christ as Savior.   

April 25, 31 AD is Crucifixion Day.   
The next day (April 26) was Passover.   50 days after Passover is Pentecost.   

There are 4 days in April after the 26th.   There are 31 days in May.   The 15th of June would be 50 days after Passover (4+31+15).   

Crucifixion Day was Nissan 14 31 AD.   Sivan 8 is 50 days after Nissan 14 (14+28+8).   

That would make the original day of Pentecost Sunday June 15, 31 AD.   

So, we should celebrate the outpouring of the Holy Spirit on June 15th each year.   

Feast - Of - Weeks