The feast of All Saints has observed by the Church at least since the fourth century. For a time it was celebrated on the Sunday following Pentecost due to the obvious link of the outpouring of the Holy Spirit upon the apostles and the foundation of the Church. Tertullian's famous insight that the Church is built on the blood of the martyrs rings true; the witnesses to the person of Jesus Christ concretizes the Christian faith and makes relevant for us the work of holiness given to us by God. In Rome, Pope Boniface IV consecrated what was the pagan pantheon as the Church of All Saints and moved the liturgical observance of All Saints to November first.
From a sermon by Saint Bernard of Clairvaux for the feast of All Saints
Why should our praise and glorification, or even our celebration of this feast day, mean anything to the saint? What do they care about earthly honors when their heavenly Father honors them by fulfilling the faithful promise of his Son? What does our commemoration mean to them? The saints have no need of honor from us; neither does our devotion add the slightest thing to what is already theirs. Clearly, when we venerate their memory, it is serving us, not them. But I tell you, when I think of them, I feel myself inflamed by a tremendous longing to be with them.
Calling the saints to mind inspires, or rather arouses in us, above all else, a longing to enjoy their company which is desirable in itself. We long to share in the citizenship of heaven, to dwell with the spirits of the blessed, to join the assembly of patriarchs, the ranks of the prophets, the council of apostles, the great host of martyrs, the noble company of confessors and the choir of virgins. In short, we long to be united in happiness with all the saints. But our dispositions change. The Church of all the first followers of Christ awaits us, but we do nothing about it. The saints want us to be with them, and we are indifferent. The souls of the just await us, and we ignore them.
Come, let us at length spur ourselves on. We must rise again with Christ, we must seek the world which is above and set our mind on the things of heaven. Let us long for those who are longing for us, hasten to those who are waiting for us, and ask those who look for our coming to intercede for us. We should not only want to be with the saints, we should also hope to possess their happiness. While we desire to be in their company, we must also earnestly seek to share in their glory. Do not imagine that there is anything harmful in such an ambition as this; there is no danger in setting our hearts on such glory.
When we commemorate the saints we are inflamed with another yearning: that Christ our life may also appear to us as he appeared to them and that we may one day share in his glory. Until then we see him, not as he is, but as he became for our sake. He is our head, crowned, not with glory, but with the thorns of our sins. As members of that head, crowned with thorns, we should be ashamed to live in luxury; his purple robes are a mockery rather and honor. When Christ comes again, his death shall no longer be proclaimed, and we shall know that we also have died, and that our life is hidden with him. The glorious head of the Church will appear and his gloried member will shine in splendor with him, when he transforms this lowly body anew into such glory as belongs to himself, its head.
Therefore, we should aim at attaining this glory with a wholehearted and prudent desire. That we may rightly hope and strive for such blessedness, we must above all seek the prayers of the saints, that what is beyond our own efforts to obtain may be granted through their intercession.
(Sermon 2; S. Bernardi Opera, ed. J. Leclercq and H. Rochais, vol. V, 1968, pp364-8; ET by ICEL)
Almighty and everlasting God, Who has given us in one feast to venerate the merits of all Thy Saints, we beseech Thee through the multitude of intercessors, to grant us the desired abundance of Thy mercy.