During the month of November the Catholic Church remembers in a special way the faithful departed, who are not as yet in Heaven, that is, the people who have left this world, but if heir souls were not totally clean, they would have to be purified in the purgatory first, before they can enter Heaven. We are all encouraged to offer prayers and sacrifices for their eternal salvation.
Bishop José Sánchez González, explains in his Ecclesia Digital article: “During the month of November, the Church invites us to think about life in its definitive stage beyond death. (…) On the second day of the month, in the Commemoration of all the faithful departed, our communion and our obligation to those who, like us, were baptized and incorporated into the Church, are not remembered and today, temporarily separated by death, they may be in need of our help through prayer and the suffrages offered to them, at the same time that they may be our intercessors before the Merciful Lord. "
Souls in Purgatory are biblical
In the II Maccabees 12: 39-46 we discover Judas Maccabeus and members of his Jewish military forces
collecting the bodies of some fallen comrades who had died in battle. When discovered that these men were carrying sacred tokens from the idols of Jamnia that the law forbids the Jews to carry "carry" (vs 40) Judas and his companions understood that they had died as punishment for this sin. Therefore, Judas and his men returned to prayer pleading that the sins, that they had committed could be totally erased. He also made a collection, and sent to Jerusalem to offer as a sin offering. In doing this, he performed very well and honourably. Therefore he made atonement for the dead, so that they freed from their sin.
Jesus Himself asks us to pray for the souls in the Purgatory
For the eighth day, Saint Faustina recorded these words of Jesus Chist:
Today bring to Me THE SOULS WHO ARE DETAINED IN PURGATORY, and immerse them in the abyss of My mercy. Let the torrents of My Blood cool down their scorching flames. All these souls are greatly loved by Me. They are making retribution to My justice. It is in your power to bring them relief. Draw all the indulgences from the treasury of My Church and offer them on their behalf. Oh, if you only knew the torments they suffer, you would continually offer for them the alms of the spirit and pay off their debt to My justice. (Diary # 1226)
Commemoration of the Faithful Departed.
The tradition of praying for the dead dates back to early Christianity, where their memory was honoured, and prayers and sacrifices were offered for them.
According to the Catholic Encyclopedia, in the early days of Christianity the names of the brothers who had left were written in the diptych (two folding tables, shaped like a book, in which the early Church used to write down the names of the living and the dead for whom to pray). Later, in the sixth century, it was customary in Benedictine monasteries to have a commemoration of the deceased members at Pentecost. In Spain, in the time of Saint Isidore (d. 636), there was a similar day on the Saturday before the Sixtieth (the sixtieth day before Easter Sunday) or before Pentecost. In Germany there was a ceremony dedicated to praying for the deceased, on October 1.
This was accepted and blessed by the Church. Saint Odilo de Cluny (d. 1048) ordered that the commemoration of all the faithful departed be celebrated annually in all the monasteries of his congregation. From there it spread among the other congregations of the Benedictines and among the Carthusians. Later, several bishops took part in this celebration.

Over the years, the Catholic Church established November 2 as the official date to commemorate the deceased faithful.