Church of the Holy Family (Grand Blanc, Michigan) - interior, stained glass, All Saints' Day, All Souls' Day, Holy Rosary, Christ the King (Nheyob/Wikimedia Commons)
To Trace All Souls Day | Fr. Brian Van Hove, S.J. | CWR Blog
All Souls Day has to do with the mystery of charity, that divine love overcomes everything, even death
As Cardinal Joseph Ratzinger once said so well, one major difference between Protestants and Catholics is that Catholics pray for the dead:
"My view is that if Purgatory did not exist, we should have to invent it." Why?Catholics are not the only ones who pray for the dead. The custom is also a Jewish one, and Catholics traditionally drew upon the following text from the Jewish Scriptures, in addition to some New Testament passages, to justify their belief:
"Because few things are as immediate, as human and as widespread—at all times and in all cultures—as prayer for one"s own departed dear ones." Calvin, the Reformer of Geneva, had a woman whipped because she was discovered praying at the grave of herson and hence was guilty, according to Calvin, of superstition". "In theory, the Reformation refuses to accept Purgatory, and consequently it also rejects prayer for the departed. In fact German Lutherans at least have returned to it in practice and have found considerable theological justification for it. Praying for one's departed loved ones is a far too immediate urge to be suppressed; it is a most beautiful manifestation of solidarity, love and assistance, reaching beyond the barrier of death. The happiness or unhappiness of a person dear to me, who has now crossed to the other shore, depends in part on whether I remember or forget him; he does not stop needing my love." 
Then Judas assembled his army and went to the city of Adulam. As the seventh day was coming on, they purified themselves according to the custom, and they kept the sabbath there. On the next day, as by that time it had become necessary, Judas and his men went to take up the bodies of the fallen and to bring them back to lie with their kinsmen in the sepulchres of their fathers. Then under the tunic of every one of the dead they found sacred tokens of the idols of Jamnia, which the law forbids the Jews to wear. And it became clear to all that this was why these men had fallen. So they all blessed the ways of the Lord, the righteous Judge, who reveals the things that are hidden; and they turned to prayer, beseeching that the sin which had been committed might be wholly blotted out. And the noble Judas exhorted the people to keep themselves free from sin, for they had seen with their own eyes what had happened because of the sin of those who had fallen. He also took up a collection, man by man, to the amount of two thousand drachmas of silver, and sent it to Jerusalem to provide for a sin offering. In doing this he acted very well and honourably, taking account of the resurrection. For if he were not expecting that those who had fallen would rise again, it would have been superfluous and foolish to pray for the dead. But if he was looking to the splendid reward that is laid up for those who fall asleep in godliness, it was a holy and pious thought. Therefore he made atonement for the dead, that they might be delivered from their sin. 
Besides the Jews, many ancient peoples also prayed for the
deceased. Some societies, such as that of ancient Egypt, were actually "funereal" and built around the practice.  The urge to do so is deep in the
human spirit which rebels against the concept of annihilation after death. Although there is some evidence for a Christian liturgical feast akin to
our All Souls Day as early as the fourth century, the Church was slow to introduce such a festival because of the persistence, in Europe, of more
ancient pagan rituals for the dead. In fact, the Protestant reaction to praying for the dead may be based more on these survivals and a deformed piety
from pre-Christian times than on the true Catholic doctrine as expressed by either the Western or the Eastern Church. The doctrine of purgatory,
rightly understood as praying for the dead, should never give offense to anyone who professes faith in Christ.
When we discuss the Feast of All Souls, we look at a liturgical commemoration which pre-dated doctrinal formulation itself, since the Church often clarifies only that which is being undermined or threatened.