This year the Feast of Shavuot, or The Day of Pentecost, is celebrated on Sunday, May 27th. Shavuot means “weeks” in Hebrew as this feast day begins on the eve of seven weeks from the second day of Passover. Christians recognize this as the day the Holy Spirit was sent down from heaven to fill and empower the first followers of Jesus Christ but Jewish people have been faithfully keeping Pentecost for over 3,500 years as Shavuot is the celebration of God giving the Torah to Moses on mount Sinai. It is also marks the end of the barley harvest and the beginning of the wheat harvest.

Specific passages from Exodus and Ezekiel are read during Shavuot along with the book of Ruth

The word Pentecost (Pentecoste) comes from an ancient Greek term Pentēkostē meaning "the fiftieth day." Pentecost is traditionally celebrated fifty days (seven weeks) after Easter Sunday. It is a very important day in both Jewish and Christian religions.

The Jewish harvest festival of Shavuot (Feast of Weeks, also called Sha·bu·oth) is a celebration of the first fruits of the wheat harvest. The festival is generally associated with the day Moses received the Ten Commandments, God's Gift of the Law, on Mt. Sinai as written in the Book of Exodus in the Old Testament.

This celebration marks the beginning of harvest activities, where "a sheaf of the firstfruits" was waved by the priest before Yahweh "on the morrow after the sabbath." During the Jewish holiday of Shavuot, it is customary to decorate synagogues with greenery because tradition holds that Mount Sinai, despite being in the wilderness of the Sinai desert, miraculously flowered and bloomed in honor of this occasion.

Pentecost or Shavuot is the fourth of seven Feasts of the Lord we read about in Leviticus 23 and it is determined by counting 50 days from the second day of Passover which is theFeast of Firstfruits, the day Jesus rose from the grave. These 50 days are referred to as “Counting the Omer” as the word omer literally means “sheaf” and refers to the first sheaves of barley brought to the Temple on the Feast of Firstfruits as a means of thanking God for the harvest.

Leviticus 23:15-16
15 And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete:
16 Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD.

For Christians, Pentecost commemorates the descent of the Holy Spirit upon the eleven Apostles and other followers of Christ after his resurrection. (Judas Iscariot had already hung himself). Christian Pentecost is celebrated on the Sunday concluding the fifty day period following Easter. These events can be found in the writings of the New Testament in the Book of Acts.
Pentecost is often referred to as the "Birthday of the Christian Church." Christians observe Pentecost as a powerful feast of the salvation, because it speaks about the giving of the Law on Mount Sinai, about the founding of the Church, and about the Final Judgment.

Pentecost is considered to be the highest ranking Great Feast of the Lord, second in rank only to Easter. The service is celebrated with an All-night Vigil on the eve of the feast day, and the Divine Liturgy on the day of the feast itself. Orthodox temples are often decorated with greenery and flowers on this feast day, and the celebration is intentionally similar to the Jewish holiday of Shavuot, which celebrates the giving of the Mosaic Law.

From a Christian standpoint, Pentecost can be seen as parallel to Shavuot, as Easter is to Passover. On Passover, the Jews were delivered from slavery in Egypt; On Easter, mankind was delivered from slavery to sin. On Shavuot, the Children of Israel received the Law; On Pentecost, the Church received the fullness of the Holy Spirit.
Since Easter doesn't fall on the same day each year, Pentecost doesn't fall on the same day each year either. And since Easter is calculated differently by the Eastern Orthodox Christian Churches and Western Christian Churches the feasts are celebrated on different dates.

In certain years, Pentecost will coincide on the same day as calculated by both churches. In the Eastern Orthodox Church, the earliest date in the year for Pentecost is May 24 and the latest possible date is June 27. In the Western Christian Church, the earliest date for Pentecost is May 10 and the latest is June 13.
For most Christians, Pentecost represents the birthday of the Church, signified by the descent of the Holy Spirit. The Pentecostal Church derives it's name from this event.