What does Boxing Day have to do with boxing?
c.1300, certeynte, "surety, pledge," from Anglo-French certeinté (late 13c.), Old French certainete "certainty," from Latin or Vulgar Latin *certanitatem (source of Old Spanish certanedad); see certain. Meaning "that which is certain" is attested from early 14c.; meaning "quality of being certain" is from mid-14c.