Early History of Memorials for Martyrs and Saints
From the earliest days of the Church, Christians venerated martyrs on the anniversary of their deaths by celebrating Mass on their tombs. Over the next few centuries relics began to be transferred between dioceses and memorials for several martyrs were celebrated in common. During the persecutions of Diocletian in the early 300's so many Christians were killed that it became impossible to create separate memorials for each so joint memorials became common.
In the late 300s St. Basil the Great sent a letter to the bishops of Pontus inviting them to celebrate a common feast in honor of the martyrs.
The earliest record of a feast honoring all Christian martyrs is from a homily of St. John Crysostom in the 407 saying that there was a feast celebrated in Constantinople.
All Saints Day in the Western Church
The first official recognition of a feast for all martyrs and the Virgin Mary comes in 609 or 610 on May 13th when St. Boniface IV concecrated the Pantheon as a church. The Pantheon had been given to the Church as a gift from Emperor Phocas. The feast coincided with the conclusion of the Pagan feast of Lemures which was celebrated to appease restless spirits.
Pope Gregory III (731-741) dedicated a chapel in St. Peter's Basilica to all saints during his reign. Louis the Pious made the celebration obligatory in the Frankish empire in 835 and Pope Gregory IV officially declared that the Feast of All Saints Day was to be celebrated by the whole Church on November 1st in 837.
The octave of All Saints was added during the reign of Pope Sixtus IV (1471-1484).
The Feast of All Saints is a holy day of obligation in the Catholic Church.
All Saints Day in the Eastern Church
In the Eastern Church the feast followed an ancient tradition of celebrating a feast for all saints on the first Sunday following Pentecost. The feast gained “official” status during the reign of the Byzantine Emperor Leo VI (886-911). He built a church for his holy wife Empress Theophano and when told he couldn't dedicate it to her, dedicated it to “All Saints” in the hope that she might some day be named a saint and therefore be celebrated in the church.
The second Sunday following Pentecost is reserved for honoring local groups of saints.
All Saints in Architecture
The Pantheon in Rome was originally a Roman temple dedicated to all the gods. In 609 the Byzantine Emperor Phocas gave the temple to Pope Boniface IV who rededicated as the church of St. Mary and the Martyrs. Many remains of Christians who originally had been buried in the catacombs were transferred to the church and placed under the altar.
All Saints in Music
Here is a traditional recitation of the Litany of All Saints