"But I got him, Father. I got him as he went by, and he didn’t look so pretty with hives all over him, did he Merope?"
-- Morfin Gaunt, speaking in Parseltongue (HBP10)
Merope Gaunt liked to wait by the window for a glimpse of handsome Tom Riddle Sr., a rich Muggle boy. Morfin Gaunt, Merope’s brother, mocked her for it and late one night he caught her at it. Just for spite, he hit Tom Riddle Sr. with a Hives Jinx. This attack brought a visit from Bob Ogden, Head of the Department for Magical Law Enforcement, the next day (HBP10).
Since Mofin was summoned to appear on September 14th, these events must have happened in early September. The year would be 1925, the year before Merope used a love potion to force Tom Riddle Sr. to marry her and father a child, who was born on December 31, 1926.