Philip Shaff on the Roman Catholic origins of All Saints Day and All Souls Day:
“In addition to the commemoration days of particular saints, two festivals were instituted for the commemoration of all the departed.
The Festival of ALL SAINTS was introduced into the West by Pope Boniface IV. on occasion of the dedication of the Pantheon in Rome, which was originally built by Agrippa in honor of the victory of Augustus at Actium, and dedicated to Jupiter Vindex; it survived the old heathen temples, and was presented to the Pope by the Emperor Phocas, A.D. 607; whereupon it was cleansed, restored and dedicated to the service of God in the name of the ever-Virgin Mary and all martyrs. Baronius tells us that at the time of the dedication on May 13 the bones of the martyrs from the various cemeteries or in solemn procession transferred to the church in twenty-eight carriages. From Rome the festival spread during the ninth century over the West, and Gregory IV. induced Lewis the Pious in 835 to make it general in the Empire. The celebration was fixed on the first of November for the convenience of the people who after harvest had a time of leisure, and were disposed to give thanks to God for all his mercies.
The Festival of ALL SOULS is a kind of supplement it to that of All Saints, and is celebrated on the day following (Nov. 2). Its introduction is traced to Odilo, Abbot of Cluny, in the tenth century. It spread very soon without a special order, and appealed to the sympathies of that age for the sufferings of the souls in purgatory. The worshippers appear in mourning; the mass for the dead is celebrated with the “Dies irae, Dies illa,” and the oft-repeated “Requiem aeternam dona eis, Domine.” In some places (e.g. in Munich) the custom prevails of covering the graves on that day with the last flowers of the season.”
~Philip Shaff, History of the Christian Church (Grand Rapids: Eerdmans,  1987), 4:445-446.