They're singing "Kyrie Eleison", which is Greek for "The Lord is Risen"
When people moved out of Africa further north to West Asia and then to Europe, about 60,000 BC, they began to live in a world with changing seasons, where it was hard to find fresh food in the wintertime. People would get pretty hungry in the last part of the winter. They were always happy in the spring when the first green plants started to grow again, and the birds began to lay eggs.
When people in West Asia began to live mainly on food they had farmed, around 9000 BC, they were even more dependent on spring coming for their food. Because they were so happy, they would have a party. And, to keep their gods happy too, they would sacrifice some of the first new food - barley - to their gods. This was the origin of the old Babylonian New Year holiday, and of the Jewish festival of Passover.
By about 500 BC, though, more and more people lived in cities, and were not farmers. Rich people, especially, were not farmers anymore. And rich people decided what to celebrate. Rich people wanted different reasons for their parties, reasons that would be more relevant to their own lives. Priests like Ezra and Nehemiah made Passover into a festival of freedom, about the Jews escaping from slavery in Egypt. But people still ate the new barley, and eggs, and lamb. And they still sacrificed some of this new food to their God.
By the time of Jesus, though, sacrifice itself was beginning to seem old-fashioned. People were killing animals for God less and less often, and Christianity gave them a good excuse to stop entirely, with the explanation that Jesus had sacrificed himself as an end to all other sacrifices. By the 300s AD, as more and more people became Christians, the point of Easter became to celebrate that one specific sacrifice - Jesus' crucifixion.
People in western Europe began to celebrate Easter every year on the first weekend after the first full moon after the first day of spring. In Eastern Europe, they added that Easter had to be after Passover as well, so Easter often came later there. People called that Friday "Good Friday", the anniversary of the Crucifixion, and then on that Sunday, Easter Sunday, they celebrated Jesus rising again from being dead and bringing with him help for everyone who believed in him.
But even so, Easter continued to come in the spring, and people still celebrated it with lamb and with eggs. The English word "Easter" comes from a Germanic word related to "East", probably the name of an early spring month, when the sun rises earlier in the east. People still knew that the time right before Easter was a time of hunger - today we call it Lent.
Learn by doing: dye some Easter Eggs
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