According to the Bible and church history, at the Pentecost the 12 apostles and other Christians that were assembled in an upper room in Jerusalem received the Holy Spirit for the first time. This caused them to be able to speak in tongues, which meant that pilgrims from many nations gathered in the city simultaneously understood them in their own languages.
After Jesus was gone, his apostles and other close followers waited in prayer in a large room for a sign he had promised them. On the day of the Jewish feast called the Pentecost, which yearly occurred 50 days after the Passover, the sound of a strong wind filled the room where the Christians waited, and flames in the shape of tongues rested on each of them. Following this anointing, they lost the fear of persecution that had been oppressing them and went out and preached to the gathered crowd. This marked the beginning of the expansion of the Christian faith.
Christians throughout the world celebrate the Pentecost 50 days after Easter, though because of the difference between western and eastern calendars, Easter and Pentecost are celebrated on different days in the West and the East. Because Pentecost always falls on a Sunday, it is a holiday in Christian countries, and in many European countries the Monday after Pentecost is a holiday as well.