The collect for today, All Saints' Day, from the 1662 Book of Common Prayer:

O almighty God, who hast knit together thine elect in one communion and fellowship, in the mystical body of thy Son Christ our Lord; Grant us grace so to follow thy blessed Saints in all virtuous and godly living, that we may come to those unspeakable joys, which thou hast prepared for them that unfeignedly love thee; through Jesus Christ our Lord. Amen.

All Saints Day is the feast of all the redeemed, known and unknown, who are now believed to be in heaven. In the Western Church it is kept on November 1. The Orthodox Churches observe it on the first Sunday after Pentecost.

All Saints Day is the modern name of what was once called in older English All Hallows Day. Hallows was a word that referred to the Saints; they were the "hallowed ones", in the same sense that we say "Hallowed be Thy name" in the Lord's prayer. The night before All Saints Day became known as All Hallow’s Even or “holy evening.” Eventually the name was shortened to the current Hallowe’en.

All Saints Day originated in the Eastern church as a commemoration of the martyrs of the whole world. St Ephrem of Syria (c. 306-373) mentions a Feast dedicated to the saints in his writings. St. Chrysostom of Constantinople (347-407) was the first Christian we know of to assign the Feast to a particular day: the first Sunday after Pentecost.

On 13 May, 609 or 610, Pope Boniface IV dedicated the Pantheon at Rome to St Mary and the Martyrs, thus consecrating it to Christian usage as a church. This used to be seen as the origin of the feast in the West, but this connection is now thought doubtful. Instead, the feast of All Saints is now traced to the foundation by Pope Gregory III (reigned 731–741) of an oratory in St. Peter's for the relics "of the holy apostles and of all saints, martyrs and confessors, of all the just made perfect who are at rest throughout the world," with the day moved to 1 November.

Egbert of York brought the feast to England. It was Pope Gregory IV (d. 844) who in 835 ordered the Feast of All Saints to be universally observed on 1 November.

The festival was retained after the Reformation in the calendar of the Church of England and in that of many of the Lutheran churches.

An appropriate prayer from the 4th-century Apostolic Constitutions:

We present the offering to you for all the saints who have been pleasing to you from the beginning, patriarchs, prophets, the just men, apostles, martyrs, confessors, bishops, priests, deacons, subdeacons, readers, singers, virgins, widows, lay people, and for all those whose name you know.

Source: The Spirit of Early Christian Thought, by Robert Louis Wilken (Yale University Press, 2003), p. 46.