Day of the Dead, celebrated by Christians on 1 November, draws on much older festivities that honor the deceased. When Pope Boniface IV first proposed a celebration in tribute to the Virgin Mary and All Saints in 610 c.e., the date chosen was 13 May to coincide with Lemuria, a Roman festival dedicated to ancestors. However, this celebration of all saints did not succeed; it was only some 200 years later that the holiday finally took hold. In 835 Louis the Pious, emperor of the Carolingian Empire, declared a celebration of the Christian saints in autumn, partly in an attempt to override the widespread pagan rituals practiced at that time of year. These festivities were likely related to the Celtic harvest festival of Samhain, when the souls of the dead were said to return to their homes. In the ninth century, under Pope Gregory IV, the Catholic Church finally mandated 1 November as All Saints Day. Next came the solemn mass initiated in 990 by Odily, abbot of Cluny, to commemorate all the dead in monasteries under his authority. This practice gradually spread throughout Europe, and 2 November was eventually established as All Souls Day in a further attempt to supplant the continuing pagan celebrations in honor of the dead.