(Source) The Church commemorates St John on this day because of the annual pilgrimage to his grave.
When St John was more than one hundred years old, he took seven of his disciples and went to a spot outside the city of Ephesus. There he told them to dig a grave in the form of a cross. Then he climbed into the grave and told his disciples to cover him with earth. Later, the grave was opened and the saint’s body was not there.
Each year on May 8 a red dust would arise from the grave which the faithful collected in order to be healed of their illnesses.
St John’s main Feast is on September 26.