I wanted to give this topic a thorough shakedown but I simply don't have the time. But for now here's an interesting morsel. Mark 9:2 and Matt 17:1 (following Mark) say that "after 6 days" Jesus took Peter James and John to the mountain for the transfiguration scene. Luke (9:28) follows his two sources fairly well here except his says " 8 days". Why?
When researching this I found a slew of rather silly attempts to harmonize the two, such as both mean "about a week so there is no discrepancy" or " Luke was a Gentile so he used a different day counting method" then ironically I found another that insisted that Luke alone was using a Jewish reckoning. But the difference appears to have made by the author of Luke for theological reasons. It is suggested by commentators that the transfiguration was here being associated with the Jewish Sabbath by Mark and Matt (perhaps mark harmonized with matt here?) and that the 'after 6 days' was allusive to the 7th day, the Sabaath with all its popularly associated prophetic significance. The span of days counting from Jesus fortelling his death till his appearance in glory on the mountain. Here is how it reads in Luke:
22( U ) saying, " ( V ) The Son of Man must suffer many things and be rejected by the elders and chief priests and scribes, and be killed and be raised up on the third day."
23 And He was saying to them all, " ( W ) If anyone wishes to come after Me, he must deny himself, and take up his cross daily and follow Me.
24 "For ( X ) whoever wishes to save his life will lose it, but whoever loses his life for My sake, he is the one who will save it.
25 "For what is a man profited if he gains the whole world, and ( Y ) loses or forfeits himself?
26 " ( Z ) For whoever is ashamed of Me and My words, the Son of Man will be ashamed of him when He comes in His glory, and the glory of the Father and of the holy angels.
27 "But I say to you truthfully, ( AA ) there are some of those standing here who will not taste death until they see the kingdom of God."
28 ( AB ) Some eight days after these sayings, He took along ( AC ) Peter and John and James, and ( AD ) went up on the mountain ( AE ) to pray.
29 And while He was ( AF ) praying, the appearance of His face ( AG ) became different, and His clothing became white and gleaming.
Luke was written later than Mark or Matt according to general consensus, at a time when Sunday had become the holy day over the Jewish Saturday. As strange as it sounds to us, Sunday was counted as the 8th day. Here's an expalnation:
Samuele Bacchiocchi, a native Italian, admits, "the irrationality of an eighth day in a seven-day week did not seem to bother the ancients" (From Sabbath to Sunday, page 278). Italians count inclusively, which may seem strange to modern Americans. Bacchiocchi (page 278) says, "an Italian will often set an appointment on a Sunday for the following Sunday not by saying, 'I will meet you a week from today,' but rather 'oggi otto-- eight days today' since both Sundays are counted. Catholics call this an octave.
There are many proofs of the importance of 8 and 8th day to 2nd-5th century early Christians. Here's some examples:
Gregory of Nazianzus(A.D. 329-389), considered Sunday as "the first day with reference to those that followed and as the eighth day with regard to those that preceded" (Oratio 44 In novam Dominicam, PG 36, 612C-613A).
Tertullian(ca. A.D. 160-ca. A.D. 225) said that Christians celebrated their Sunday festival "every eighth day," meaning every Sunday.
Origen "The resurrection of the Lord is celebrated not only once a year but constantly every eight days" (Homilia in Isaiam, 5, 2).
Epistle of Barnabas 15:9 (contemporary with G. Luke) Wherefore also we keep the eighth day for rejoicing, in the whichalso Jesus rose from the dead, and having been manifested ascended into the heavens.
The 8th day (Sunday) was symbolic of a NEW week, a new start. The resurrection of Jesus was felt a NEW beginning a new life etc. The Gospel John makes explicit the connection at John 20:26 :
26 After eight days His disciples were again inside, and Thomas with them. Jesus came, the doors having been shut, and stood in their midst and said, " (AO) Peace be with you."
It is impossible to say for certain, but it seems rather strongly suggested that the author of Luke altered his sources Mark,Matt at this point because of their implicit association of the Sabbath and the resurrection. He simply updated his material following the then Gentile Christian weekly holy day and alluded to 8 rather than 6 days.