"The name's Taboo! I told you, Harry, I told you, we can't say it anymore -- we've got to put the protection back around us -- quickly -- it's how they find --"
-- Ron Weasley, cut off by the Sneakoscope coming to life (DH22)
After receiving an emotional high from hearing familiar voices and support on Potterwatch, Harry accidentally lets slip Voldemort’s name, which is taboo to speak. It breaks all of their protective enchantments and brings a group of Snatchers, led by werewolf Fenrir Greyback, to the Trio’s location. An article in the Daily Prophet tips off the Snatchers that they have caught Hermione Granger “who is known to be traveling with Harry Potter” (DH 23). Wanting to reap the 200,000 Galleon reward for Harry’s capture Fenrir decides that they must bring their prisoners, which include Dean Thomas and Griphook the goblin, to Malfoy Manor so that the Death Eaters can summon Lord Voldemort.
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.