Brewer's: Whip-dog Day
October 18 (St. Luke's Day). Brand tells us that a priest about to celebrate mass on St. Luke's Day, happened to drop the pyx, which was snatched up by a dog, and this was the origin of Whip-dog Day.
(Popular Antiquities, ii. 273.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894