I'm reading "A hunuger for God: The Sacred Discipline of Fasting in the Orthodox Church" by Fr. Peter A. Chambers. In part 1, Chapter 4 "The Development of Fasting in the Early undivided Church", starting on page 46:
The text of the Teaching of the Twelve Apostles, written in the early second century, says, "Let not your practice be with the hypocrites who fast on Monday and Thursday, for we Christians fast on Wednesday and Friday"(8,1(. ... The Apostolic Constitutions, compiled somewhat later, also point out the practice of Fasting on Wednesday because of the betrayal of the Lord, and on Friday because of his passion and death (7,23; 5,14). Many of the Fathers of the Church repeat and confirm this early tradition of obligatory fasting for all the Christian faithful. In fact, the 69th Apostolic Canon imposes a severe sanction for those who do not observe the Wednesday and Friday fast: discharge from their clerical duties for clergy and excommunication for the laity.
... We do not know how the earliest Christians observed this pious discipline of Fasting on Wednesday and Friday, but judging from a somewhat later liturgical practice of the Church, we can assume that a strict total fast was kept for the better part of the entire day. Normally, the fast ended after the ninth hour of prayer (three o'clock in the afternoon). This probably meant that in the beginning only one meal was taken on each of these two days, since this came to be the established pattern in the life of the early Church and particularly later in the monasteries. The meal was usually very frugal consisting of uncooked vegetables, fruit and bread and water. This strict rule in certain regions requ8ired that even olive oil and wine were to be excluded from the meal on these two days... This earliest tradition of Fasting on Wednesday and Friday has survived in the Orthodox Church to the present time.
Fasting before Pascha: The development of Holy Week and Great Lent
... The Apostolic Constitutions associate the establishment of the earliest Fast of Pascha [last day of Lent] with the words of the Lord about his disciples fasting "when the bridegroom will be taken away from them"... At first this time of Fasting before Pascha was neither long nor the same throughout the various regions of the ever growing and expanding Church... This fast of sorrow at the absence of the Bridegroom was a total fast from any food and drink for one or two days. Regional churches were apparently free to establish their own rules and practices, and at first there was no absolute uniformity. With the passage of time, the two-day fast was extended, and by the middle of the third century, it included the entire week before Pascha. There was also a variety in the manner of Fasting in the various regions. Thus, the practice of Fasting every Wednesday and Friday of each week, and at least a week before the celebration of Pascha each year was soon well established within the apostolic and post-apostolic age of the early Church.
A reference in the 5th Canon of the First Ecumenical Council in Nicaea makes it clear that by the year 325 AD a forty day fast before Pascha had already become a well-established tradition throughout the entire Church... Other similar historical references from the same period make it clear that there was by now a period of Fasting and a spiritual preparation for Pascha that was being associated with the forty day fasting of the Lord and of the prophets Moses and Elijah. As with the initial Fasting immediately before Pascha, so also with the Fasting of Great Lent, there was considerable variety from place to place in the duration and in the manner of the Fast until its final definitive development and determination. What can be supported with certainty is the fact that Great Lent, once fully developed, became and remained a very strict Fasting period from those earliest beginnings to the present time in the life of the Church.
As the Fasting period before Pascha increased from several days to the entire Holy Week and then gradually to a forty day Fast on top of that, the initial total fast of only several days was naturally replaced by a dietary discipline that excluded certain foods, such as meats and fish and other animal products, such as milk, cheese and eggs... The focus was clearly on the ascetic spirit that characterized the Christian way of life from the very beginning, and particularly after the fourth century when the persecutions had ended and large numbers of people were coming into the life of the Church, who needed to be instructed and guided in the truly new Christian faith and way of life.
... Lent was no longer only a preparation for the celebration of Pascha. As the weeks of Lent were extended, this period of time in the liturgical year of he Church assumed the critical purpose of making the final preparations for the large number of catechumens who were seeking to become Christians... In the meantime, the strict rules of Fasting and the spiritual discipline that had been extended for the benefit of the new catechumens, were now also being offered to and embraced and practiced by all the faithful, and not just by the catechumens.
... the treatment of each Saturday and Sunday.... These Saturday and Sundays were not to be strict Fasting days. In fact, the rule required that the Fast be somewhat suspended during these two days by simply adding something special, such as olive oil and wine, to the otherwise strict fasting food meals...
The Fasting period for Christmas
.... St. John Chrysostom refers to the Birth of Christ as "the metropolis of all the feasts" and, writing in 386, indicates that "it is not yet ten years since this feast day has become known among us", that is, as being celebrated on December 25th and separate from the feast of Epiphany on January 6th... With the passage of time, the new celebration of Christmas on December 25 acquire ever growing significance in the mind of the Church.... the Church to formally establish another annual period of fasting sometime during the 6th century ... gradually extended to become forty days -- from November 15 to December 25...
The Fast of the Holy Apostles
... Evidential traces for the origin of this Fast go back to St. Athanasios and the Apostolic Constitutions (5,20), where an apparently early period of Fasting in the life of the Church was associated with the period after Pentecost, which called for a festal period first and then a week of fasting. This one week Fast ... developed over time and was extended to June 29th, thus becoming the Fasting period before the Feast of the Holy Apostles.
The Fifteen Day Fast of August
[this Fast] also had its beginnings in the practice of the early Church. Originally this period of Fasting was divided into two sections. [The fest of the Transfiguration of the Lord and the Dormition of the Theotokos] by the ninth century, the two fasting periods,s were combined into one, thus becoming the Fast of Fifteen Days of August, or the Fast of Dekapentaugustos.