All Saints Day.

"Master William of Auxerre putteth six reasons, in the sum of the office, wherefore it was established that we should here in this world make solemnity of the saints. The first is, for the honour of the divine majesty, for when we do honour the saint or saints, we honour God in his saints, and say that he is marvellous in them. For who that doth honour to saints, he honoureth him specially which hath sanctified them. The second is to have aid in our infirmity, for by ourselves we may have none health, therefore have we need of the prayers of saints, and therefore we ought to honour them, that we may deserve that they aid and help us. It is read in the Third Book of Kings, of the first chapter, that Beersheba is as much to say as the pit of filling, which is to say the church triumphant saying to her son, that is to say that to the church triumphant he had obtained the reign by his prayers. The third cause is for the augmentation of our surety, that is to say for the glory that is purposed in us; in their solemnity our hope and surety be augmented and increased. And if mortal men and dead might thus be enhanced by their merits, it is truth that the might and puissance shall nothing thereby be minished ne lessed. The fourth for the example of us following. For when the feast is remembered, we be called to ensue and follow them. So that by the example of them we despise all earthly things, and desire celestial things. The fifth is for the debt of interchanging neighbourhood, for the saints make of us feast in heaven. For the angels of God and the holy souls have joy and make feast in heaven of a sinner that doth penance, and therefore it is right when they make of us feast in heaven, that we make feast of them in earth. The sixth is for the procuration of our honour, for when we honour the saints we procure our honour, for their solemnity is our dignity, for when we worship our brethren we worship ourselves, for charity maketh all to be common, and our things be celestial, earthly and perdurable."

From the Golden Legend.