“It was about eight days after he said this” is how Luke introduces the climb to the mount of transfiguration. “This” refers to accepting his cross. Think about how perplexing this was to the disciples. 

“If anyone wishes to come after me, he must deny himself and take up his cross daily and follow me.”

It was not guaranteed, much less ponderable, that Jesus would die on a cross. Stoning was more likely and almost happened. They dropped their packs and busied themselves with camp. When it happened the disciples had to be roused from sleep. If Jesus were engaged in conversation with Moses and Elijah, did he acknowledge Peter’s idea for building the three tents? In the end the only one speaking to them was the Father. “Listen to him.” Later Jesus, back to normal, says “do not speak of this. Keep it to yourself.” The “eighth day” is the day that will last forever. Christ will come again. Meanwhile their destiny is in the world down below, boiling in complexity. As they walk some of that rare light leaks out.
—Fr. Andrew