Kendra Dumbledore, Albus's mother, dies early in the Summer of 1899 due to an accidental explosion conjured by his sister Ariana.
Albus Dumbledore becomes extremely close with Grindelwald during the Summer of 1899. There's a LOT that goes on during these brief few months. Dumbledore and Grindelwald become obsessed with the Deathly Hallows and even plan to start a revolution where wizards would rule the world.
The Dumbledore brothers and Grindelwald get into a three-way duel after the latter inflicts a Cruciatus Curse on Aberforth. Ariana dies while trying to intervene.