Easter, the Solemnity of the Easter Feast, does not begin until Easter Day with the resurrection, and lasts for eight days. These eight days make up the Easter Octave, (from Latin ‘octave’ = eight). There are thus two Sundays in Easter, Easter Sunday and White Sunday.
This second Sunday was called Dominica in albis - White Sunday. All those who were baptized on Easter Day wore white clothes the whole of the following week and did not dress in ordinary clothes until after White Sunday.
This Sunday is also called St. Thomas’ Sunday because the gospel readings are about the Doubting Thomas. The event where he would not believe until he had placed his fingers in Jesus’ wounds took place eight days after the resurrection, on White Sunday.
Celebration of octaves in connection with solemn feast days developed from the 4. – 7. centuries. Octaves were celebrated for Easter, Pentecost and Christmas among others, and in medieval times octaves were celebrated for important saints. The number eight became a central symbol for eternity, and burial churches were often built as octagons. The one closest to us here in Trondheim is the octagon in Nidaros Cathedral, which was built to encircle the grave of St. Olav.
Pope John Paul II declared in 2000 that White Sunday should be celebrated as Divine Mercy Sunday.