This passage occurs as the Transfiguration passage for the RCL Year B, most recently February of 2015.
Summary: The key to understanding this story is the number six (in Greek, "hex"). In the Bible, six connotes imperfection; Jesus even dies on the sixth day. Mark says these events took place after six days and like everything else on the sixth day, it might be wonderful but it is incomplete. In this story we have incomplete disciples (in number and maturity); incomplete atonement; incomplete ministry of Jesus; if not the law and the prophets themselves revealing their limits as unable to raise the dead. The whole story is a foreshadowing for the cross and resurrection.
For this weeks "key words" I have focused on OT connections! Take your pick: Exodus or Genesis. It is all there...
εξ ("six"; there is a rough breathing mark, making it "hex" as in "hexagon"; 9:2). This is the only time Mark records something as happening "six" days later. So what happens on the sixth day? Well, on the 6th day Jesus died on the cross! Recall the OT: On the sixth day humanity was created. Very good (like Transfiguration). But final? No.
σκηνη ("tent"; 9:5) As a child, I heard the word "tabernacle" with a bit of religious awe. It simply means a tent made into a temple where God dwelt. At the end of Exodus, you can read about the Tabernacle and the "tent" presence of God, which hosted God's glory. You can go in all sorts of directions here: Peter wants to start up old-time religion here; Peter wants to pin Jesus down; Peter, well, just doesn't know what to do.
αγαπητος ("beloved"; 9:7) This harkens back to another mountain scene, where Abraham takes his beloved son up a mountain to sacrifice him. Actually, when it says Jesus "led" his disciples up the mountain (αναφερω (9:2)), the word also means sacrifice. It is the same as the word used in Genesis 22, as in Abraham leading Isaac up the mountain to be sacrificed. There is a subtle play on the Old Testament idea of sacrificing beloved sons on a mountain here; but again, this story is all about being incomplete...
One other little note of foreshadowing:
λευκος ("white"; 9:3) We will not see white again until the resurrection garden with the angels!
Grammar question: Does anyone know why the word "we" (ημας) in 9:5 is in the accusative and not nominative? The English translators leave it in the accusative by making it "it is good for us to be here" but in this is not really what is going on in the Greek. In the Greek, the word ημας is the subject of the infinitive phrase, "we to be" and in Greek the subject of infinitive phrases takes the accusative.