"He changed, Harry, he changed! It's as simple as that! Maybe he did believe these things when he was seventeen, but the whole of the rest of his life was devoted to fighting the Dark Arts!" -- Hermione Granger
Hermione gives Harry a copy of The Life and Lies of Albus Dumbledore that she picked up in the Bagshot house. They learn that the young man in the picture was Gellert Grindelwald, who according to Rita Skeeter was very close to Albus Dumbledore for a brief period when they were young, at a crucial period in both their lives.
Calendar and Dates
The day after the Christmas Eve visit to Godric's Hollow, so this all takes place on Christmas Day, 1997.
Interesting facts and notes
he had lost all the bones in his right arm once
Thanks to Gilderoy Lockhart's clumsy attempt to fix the arm when it had been broken by a rogue Bludger (CS10).
scars to his chest and forearm
From where Hermione had to use a Severing Charm to detach the Horcrux from his chest and from Nagini's bite, both of which occurred the previous day (DH17).
to join those on his hand
That would be "I must not tell lies" (OP13).
Now approaching his eighteenth birthday
This puts a limit on when Dumbledore's birthday might be - he would have been 17 at the beginning of the school year (presumably September) but this indicates that he was not yet 18 at the end.
Elphias "Dogbreath" Doge, the dim-witted but devoted sidekick he had pickd up at school
Gee, Rita, you wouldn't be holding a grudge that he "refused to be interviewed for this book", would you?
the effort I put into procuring Veritaserum
It's ridiculous that this isn't illegal, let alone that Rita should feel perfectly safe in boasting about it in a book.
her great-nephew, Gellert Grindelwald
This doesn't, unfortunately, indicate on which side of the family they were related - whether through the Grindelwald side or through Gellert's mother's side.
the thoughts of their seventeen-year-old-hero
So Albus Dumbledore was still seventeen years old while Gellert was sixteen, although Albus had just completed his seventh year at Hogwarts. He must have been nearly eighteen by then, because he would have had to be seventeen by September first when the school year had begun. So the two were within two years of one another's age, closer in age than Aberforth and Albus were.
for more, see chapter 22
So "The Greater Good" is one of the first 21 chapters of the book, and chapter 22 presumably focuses on the duel and possibly the events leading up to it.
Dumbledore delayed, for some five years of turmoil, fatalities, and disappearances, his attack upon Gellert Grindelwald
So Grindelwald's open rise to power began at the latest in 1941, since the duel took place sometime in 1945.
Exceptional character moments
Albus Dumbledore's friendship and plans with Grindelwald
Hermione being so broken up over the damage to Harry's wand
"Dumbledore intended, next, to take a Grand Tour with Elphias 'Dogbreath' Doge, the dim-witted but devoted sidekick he had picked up at school."
"I don't know who he loved, Hermione, but it was never me. This isn't love, the mess he's left me in. He shared a damn sight more of what he was really thinking with Gellert Grindelwald than he ever shared with me."
"Anyways, it's - it's an awful thought that Dumbledore's ideas helped Grindelwald rise to power."
"Dumbledore was the one who stopped Grindelwald, the one who always voted for Muggle protection and Muggle-born rights, who fought You-Know-Who from the start and who died trying to bring him down!"