The feast of St. Martin is November 11. His Martinmas will come, as it does to every hog- i.e. all must die.
November was the great slaughter-time of the Anglo-Saxons, when beeves, sheep, and hogs, whose store of food was exhausted, were killed and salted. Martinmas, therefore, was the slaying time, and the proverb intimates that our slaying-time or day of death will come as surely as that of a hog at St. Martin's-tide.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894