‘And it happened about eight days after these sayings, that he took with him Peter and John and James, and went up into the mountain to pray.'
It will be noted that Luke has changed ‘after six days' to ‘about eight days after'. There is no problem with this mathematically for six whole days could, when taking into account part days, (which was quite normal), be the equivalent of ‘eight days'. But we may ask, why the alteration? It will be noted that Luke has twice previously referred to an eight day period, once in 1. 59 when they came to circumcise John the Baptiser on the eighth day, at which point he was to be named, and once in Luke 2:21 where we read, ‘and when eight days were fulfilled for circumcising Him they called His name Jesus'. Each eight day period resulted in a naming. Perhaps the thought here then is that eight days after the declaration of Him as Messiah (Luke 9:20), or eight days after the first revelation of the fact that He will come in His glory (Luke 9:26), He is revealed in His glory and named by God as His Son and Chosen One, indicating sealing following a kind of ‘birth'. ‘These sayings' could certainly be seen as including Peter's declaration of faith and what followed, and even more certainly contain the declaration about His coming in glory.
In the course of this sealing He took Peter and John and James with Him up into a mountain to pray. We can compare how Moses previously took Joshua with him when he too went into a mountain to see the glory of God and to pray, and to receive from God His revelation in the Law. Here then is the preparation for a new revelation from God. Interestingly each time Jesus takes these three apart it is in order that they might hear significant words, firstly in the raising from the dead of Jairus' daughter (‘child arise'), secondly here (‘this is My Son, My Chosen, listen to Him'), and thirdly in Gethsemane, (‘Father' if you are willing, remove this cup from Me, nevertheless not My will but Yours be done'). The first reveal His power as the Resurrection and the Life (John 11:25), the second His true Sonship and Destiny (Luke 3:22; John 1:14; John 1:18), and the third His obedience unto death (compare Philippians 2:8; Hebrews 10:5-10), all central to His work.