For an explanation of the present Feast and understanding of its truth, it is necessary for us to turn to the very start of today's reading from the Gospel: "Now after six days Jesus took Peter, James and John his brother, and led them up onto a high mountain by themselves" (Mt.17:1). First of all we must ask, from whence does the Evangelist Matthew begin to reckon with six days? From what sort of day is it? What does the preceding turn of speech indicate, where the Savior, in teaching His disciples, said to them: "For the Son of Man shall come with his angels in the glory of His Father," and further: "Amen I say to you, there are some standing here who shall not taste death, until they have seen the Son of Man coming in His Kingdom" (Mt.16:27-28)? That is to say, it is the Light of His own forthcoming Transfiguration which He terms the Glory of His Father and of His Kingdom. The Evangelist Luke points this out and reveals this more clearly saying: "Now it came to pass about eight days after these words, that He took Peter and John and James, and went up the mountain to pray. And as He prayed, His countenance was altered, and His raiment became a radiant white" (Luke 9:28-29). But how can the two be reconciled, when one of them speaks definitively about the interval of time as being eight days between the sayings and the manifestation, whereas the other (says): "after six days?" There were eight on the mountain, but only six were visible. Three, Peter, James and John, had come up with Jesus, and they saw Moses and Elias standing there and conversing with Him, so altogether there were six of them. However, the Father and the Holy Spirit were invisibly with the Lord: the Father, with His Voice testifying that this was His Beloved Son, and the Holy Spirit shining forth with Him in the radiant cloud. Thus, the six are actually eight, and there is no contradiction regarding the eight. Similarly, there is no contradiction with the Evangelists when one says "after six days," and the other says "eight days after these words."
Source: Church Calender