From Franciscan Media:
The earliest certain observance of a feast in honor of all the saints is an early fourth-century commemoration of “all the martyrs.” In the early seventh century, after successive waves of invaders plundered the catacombs, Pope Boniface IV gathered up some 28 wagon-loads of bones and reinterred them beneath the Pantheon, a Roman temple dedicated to all the gods. The pope rededicated the shrine as a Christian church. According to Venerable Bede, the pope intended “that the memory of all the saints might in the future be honored in the place which had formerly been dedicated to the worship not of gods but of demons” (On the Calculation of Time).
But the rededication of the Pantheon, like the earlier commemoration of all the martyrs, occurred in May. Many Eastern Churches still honor all the saints in the spring, either during the Easter season or immediately after Pentecost. How the Western Church came to celebrate this feast, now recognized as a solemnity, in November is a puzzle to historians. The Anglo-Saxon theologian Alcuin observed the feast on November 1 in 800, as did his friend Arno, Bishop of Salzburg. Rome finally adopted that date in the ninth century.
From Catholic Straight Answers:
Both the Feast of All Saints and the Feast of All Souls evolved in the life of the Church independently of paganism and Halloween. However, elements of pagan practices were perhaps “baptized” by some cultures or attached themselves to the celebration of All Saints and All Souls.
In the West, a commemoration for all the saints also was celebrated on the first Sunday after Pentecost. The primary reason for establishing a common feast day was because of the desire to honor the great number of martyrs, especially during the persecution of Emperor Diocletion (284-305), the worst and most extensive of the persecutions. Quite simply, there were not enough days of the year for a feast day for each martyr and many of them died in groups. A common feast day for all saints, therefore, seemed most appropriate.
From Office Holidays:
All Saints’ Day is generally celebrated on 1st November as a commemoration day for all Christian saints. It may also be known as All Hallows‘ Day, Solemnity of All Saints, Hallowmas, or Feast of Saints.
The origin of All Saints’ Day may date back to a Greek Christian tradition from the 4th century, when a festival was held to honor saints and martyrs on the Sunday following Pentecost. The first recorded All Saints’ Day occurred on 13 May 609 CE when Pope Boniface IV accepted the Pantheon in Rome as a gift from the Emperor Phocas. The Pope dedicated the day as a holiday to honor the Blessed Virgin and all the martyrs.
From Learn Religions:
The current date of November 1 was instituted by Pope Gregory III (731-741), when he consecrated a chapel to all the martyrs in Saint Peter’s Basilica in Rome. Gregory ordered his priests to celebrate the Feast of All Saints annually. This celebration was originally confined to the diocese of Rome, but Pope Gregory IV (827-844) extended the feast to the entire Church and ordered it to be celebrated on November 1.
Despite concerns among some Christians (including some Catholics) in recent years about the “pagan origins” of Halloween the vigil was celebrated from the beginning — long before Irish practices, stripped of their pagan origins (just as the Christmas tree was stripped of similar connotations), were incorporated into popular celebrations of the feast. In fact, in post-Reformation England, the celebration of Halloween and All Saints Day were outlawed not because they were considered pagan but because they were Catholic. Later, in the Puritan areas of the Northeastern United States, Halloween was outlawed for the same reason, before Irish Catholic immigrants revived the practice as a way of celebrating the vigil of All Saints Day.