Pentecost, also known as Whitsunday, is a Christian observance that takes place fifty days after Easter. It has its origins in the Judaism tradition of the Festival of Weeks and commemorates the descent of the Holy Spirit onto Jesus's Apostles after his Ascension.
It is not a public holiday in the United States, but as it falls on a Sunday, most offices and schools are closed for the day.
Before Jesus, the Israeli people had a celebration called the Feast of Weeks, which was observed fifty days after Sabbath or The Feast of Firstfruits, This was a harvest festival, that would commemorate the blossoming and harvesting of wheat, and where the crops of Jerusalem were harvested. The celebrations also included a big feast. Jews from around the world would travel to Palestine during the Feast of Weeks to observe the celebration.
In Christian Tradition
Christians celebrate Pentecost, not to commemorate the Spring's harvests, but rather the Holy Spirit descending on the Apostles.
According to Acts 2 of the New Testament in the Bible, after his Crucifixion, Jesus was resurrected and spent forty days with his Apostles teaching them the word of God. Before ascending to heaven he told his disciples that they would be visited by the power of the Holy Ghost. In Jerusalem, the apostles gathered to feast and commemorate the Feast of Harvest, when the Holy Spirit descended upon the house where they were eating and into each one of them. This resulted in several people converting to Christianity, after witnessing the works of the Holy Spirit, and thus, the tradition of Pentecost in Christian Church was born.
Many see this instance, the first Pentecost, as the foundation of the modern church as we know it. It was because of the Holy Spirit that Jesus's disciples were able to spread the word of God through different countries.
How it is observed
The main symbol for Pentecost is the color red. Altars are adorned in this color, and the priests wear vestments of the same color. The red symbolizes the tongues of fire of the Holy Spirit, as well as joy.
During Pentecost there are also many baptisms, a tradition that comes from the fact that many converted to Christianism after the first Pentecost.