Since 1567 Siena has celebrated a particular religious festival known as Domenica in Albis, so called because this day saw those who had been baptised on Easter Saturday divest themselves of white clothes. According to tradition, the Consorteria delle Compagnie Laicali chose a venerated icon to sit at the heart of the old Sienese state: on the Saturday before Domenica in Albis this image is carried through the Duomo, and the festival proper begins on Sunday with Mass celebrated by the Archbishop. The evenings after this, believers make a pilgrimage to the Duomo to attend Vespers and to pay homage to the sacred image.

Source: comune.siena.it