The Didascalia Apostolorum, dated to AD 230, prescribe activities during Holy Week, including a fast on Friday:
Therefore you shall fast in the days of the Pascha from the tenth, which is the second day of the week; and you shall sustain yourselves with bread and salt and water only, at the ninth hour, until the fifth day of the week. But on the Friday and on the Sabbath fast wholly, and taste nothing. [v. 19] You shall come together and watch and keep vigil all the night with prayers and intercessions, and with reading of the Prophets, and with the Gospel and with Psalms, with fear and trembling and with earnest supplication, until the third hour in the night after the Sabbath; and then break your fasts. (v18–19)
Something similar is related in the Apostolic Constitutions (dated to 375–380) for Friday, the sixth day of the week:
But He commanded us to fast on the fourth and sixth days of the week; the former on account of His being betrayed, and the latter on account of His passion. But He appointed us to break our fast on the seventh day at the cock-crowing, but to fast on the Sabbath-day. Not that the Sabbath-day is a day of fasting, being the rest from the creation, but because we ought to fast on this one Sabbath only, while on this day the Creator was under the earth. (5.3.15)
The Pilgrimage of Etheria (or Egeria), dated to a few years later, describes a more elaborate celebration, with five main stages:
- Service at daybreak
- "And when they arrive before the Cross the daylight is already growing bright."
- Column of the flagellation
- "they all go at once with fervour to Sion, to pray at the column at which the Lord was scourged."
- Veneration of the cross
- "The casket is opened and (the wood) is taken out, and both the wood of the Cross and the title1 are placed upon the table. Now, when it has been put upon the table, the bishop, as he sits, holds the extremities of the sacred wood firmly in his hands, while the deacons who stand around guard it."
- Station before the cross
- "Thus from the sixth to the ninth hours the lessons are so read and the hymns said, that it may be shown to all the people that whatsoever the prophets foretold of the Lord's Passion is proved from the Gospels and from the writings of the Apostles to have been fulfilled."
- Evening offices
- "And after the dismissal at the martyrium, they go to the Anastasis, where, when they arrive, the passage from the Gospel is read where Joseph begged the Body of the Lord from Pilate and laid it in a new sepulchre."
The excerpts above are taken from the full text of The Pilgrimage of Etheria, which is too lengthy to quote here in full. The Encyclopedia of Ancient Christianity briefly summarizes:
Christians spent the time reflecting upon Jesus’ morning appearance before Pilate (Mk 15:2-15 and parallel texts) and the flagellation (Mk 15:16-20 and parallel texts). At noon, the wood of the cross found by Helena was shown to the faithful; then, for three hours, the people read the account of the passion, along with OT prophecies about Christ.