Whitsun is celebrated on the seventh Sunday after Easter, and it commemorates the Holy Spirit descending on the Apostles and Christ's followers at Pentecost:
"When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. When they heard this sound, a crowd came together in bewilderment, because each one heard their own language being spoken. Utterly amazed, they asked: “Aren’t all these who are speaking Galileans? Then how is it that each of us hears them in our native language? Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome (both Jews and converts to Judaism); Cretans and Arabs—we hear them declaring the wonders of God in our own tongues!” Amazed and perplexed, they asked one another, “What does this mean?”
Some, however, made fun of them and said, “They have had too much wine.”" (Acts 2: 1-13 New International Version)
In Tudor times, it was traditional for communities to come together for a "church ale", a festival which aimed to raise fund for the church. Ale would be brewed for the occasion, and there would be food and entertainment such as Morris dancing and archery competitions. Attendees were expected to make a donation or ale would be sold.
Picture: A medieval illustration of Pentecost from the Hortus deliciarum manuscript of Herrad of Landsberg.