They discover the Cup when they break into Gringotts with the help of Polyjuice Potion and some well-placed Imperious Curses.
The Trio recover Hufflepuff’s Cup, a Horcrux, from the Lestrange Family Vault
Date May 1st, 1998
Certainty Stated in canon
Location in Canon DH26: Gringotts
Type of Event Wizarding world