“Kill me, then. Voldemort, I welcome death! But my death will not bring you what you seek ... There is so much you do not understand ... Kill me, then! You will not win, you cannot win! That wand will never, ever be yours—"
-- Gellert Grindelwald's last words (DH23)
Voldemort traces the Elder wand to Grindelwald, who he then visits in a lonely cell atop the tallest tower of Nurmengard. When Grindelwald defies Voldemort, the Dark Lord murders him with a Killing Curse (DH23).
Timeline Notes
Harry "sees" this happening through his connection with Voldemort at the same time that he, Ron, and Hermione are captured by Snatchers.
The real world calendar for 1998 gives us a date of April 12 for Easter. Narcissa states that Draco is at Malfoy Manor for the Easter holidays, so these events happened during a two week period including April 12. The books also state that the Trio listens to Potterwatch in March:
But not until March did luck favor Ron at last. Harry was sitting in the tent entrance, on guard duty, staring idly at a clump of grape hyacinths that had forced their way through the chilly ground, when Ron shouted excitedly from inside the tent. “I’ve got it, I’ve got it! Password was ’Albus’! Get in here, Harry.” (DH22)
The only way that the capture by Snatchers in March can happen during two week period including April 12 is if those two weeks begin on March 29. Therefore the capture by Snatchers happened on the evening of March 29, 30, or 31. For this timeline, we'll assign the approximate date of March 30.