Katie falls victim to the Imperius Curse and is forced to carry a package into Hogwarts from the village. The package, intended to be a deadly gift for Dumbledore, contains a cursed opal necklace. While walking, the packaging slips off and Katie’s hand brushes the necklace. She is immediately struck by the curse and begins shrieking and levitating six feet off the ground (HBP12).
The visit to Hogsmeade during which Katie was cursed took place on a Saturday in October. If the actual calendar of that year is used, this would mean that Katie was touched the cursed necklace on either October 12 or 19.