The commemoration of All Saints was first made universal in 835 AD by the Emperor Louis the Pious, in the 21st year of his reign, at the suggestion of Pope Gregory IV. This information reaches us through the 12th century Chronographia or Chronicle of Sigebert of Gembloux, who records the following entry for the year:
835. / 21 / 5. Monente Gregorio papa et omnibus episcopis assentientibus, Ludowicus imperator statuit, ut in Gallia et Germania festivitas omnium Sanctorum in Kalendis Novembris celebraretur, quam Romani ex instituto Bonefacii papae celebrabant. Hoc tempore reliquiae Viti martyris a Parisius ad Corbeiam Saxoniae transferuntur; unde ipsi Franci testati sunt, quod ab illo tempore gloria Francorum ad Saxones translata sit. Ebbo Remorum archiepiscopus deponitur; aliique inulti, qui cum eo in deiectionem Ludowici imperatoris conspiraverant, damnantur et exiliantur.
At the suggestion of Pope Gregory and with the agreement of all the bishops, the emperor Ludovicus ordered that in Gaul and Germany the festivity of the all the Saints would be celebrated on the Kalends of November, which the Romans were celebrating by the institution of Pope Boniface. At this time the relics of Vitus the martyr are transported from Paris to Corbie in Saxony; because of which the Franks themselves bore witness that from that time the glory of the Franks was transferred to the Saxons. Ebbo, archbishop of Riems is deposed; and others unpunished, who had conspired with him to depose the emperor Ludovicus, are condemned and exiled.
This made it official throughout the Holy Roman Empire. No doubt the modern celebration results from this edict.
The reference to Boniface IV is slightly misleading. The Romans were not celebrating on 1 November. The reference is to the Liber Pontificalis and the year 607, when Pope Boniface IV dedicated the Pantheon to St Mary Ever-Virgin and All the Martyrs. This dedication took place and is commemorated, not on November 1, but on May 13, according to the old Roman martyrology. But that is another post!