Jungmann, Josef A., S.J. "Chapter Three: Sunday and Easter in the Primitive Church." The Early Liturgy to the Time of Gregory the Great. (translated by Francis A. Brunner, C.S.S. R., Notre Dame, Indiana: University of Notre Dame Press, 1959, pp. 19-28.
Jungmann observes that the earliest Christians retained the idea of a seven day week byt transferred the emphasis from the Sabbath to the first day. He cites 1 Corinthians 16:2 as an example of Christians using the Sunday as the foundation of their calendar (Jungmann 1959, 19). The Sunday was not considered a replacement Sabbath, but the Lord's day, the celebration of Christ's resurrection. Didache 14.1 makes it clear that was the day for meeting and receiving the Eucharist (Jungmann 1959, 20). Jungmann illustrates the concept of a Lord's day being Sunday and no other day.
Jungmann notes a tendency in early Christianity to call Sunday the eighth day, thus avoiding the idea that the Sabbath, as the last day of the week, was the climax (Jungmann 1959, 22). Church fathers then proceeded to see the number eight as a sign of resurrection and renewal. The pagan reference to the day of the sun, while not approved by Christians, was soon taken as a reminder that Christ is the sun of righteousness who rises and brings light and life (Jungmann 1959, 24).
Jungmann contends that Easter was early recognized as coming from apostolic practice. He sees it as the direct continuation of the Passover (Jungmann 1959, 25). The debate about it in the second century was based on whether the date of 14 Nisan was the proper day or if it should be celebrated on the Sunday afte rthat date (Jungmann 1959, 25-26). Jungmann describes the challenge caused by the interaction of the Jewish lunar calendar and the Julian calendar. In the end, the church, at least in the West, celebrates Easter the Sunday after 14 Nisan.
The Easter celebration was divided into two portions, the passion and resurrection. The first part, primarily penitential, began on Friday. On Sunday morning, the celebration of resurrection began with the reception of the Eucharist (Jungmann 1959, 27). The celebration then continued with no fasting through the fifty days until Pentecost.
The period of fasting before the celebration of Easter developed after the recognition of Pentecost. Jungmann bases it on the idea that the disciples would fast when the Bridegroom, Christ, is away. Having calculated this to be 40 hours, the Christian community took up a 40 day fast time before Easter for special devotion, and the 40 hours before the resurrection in complete fasting (Jungmann 1959, 28).