What does Boxing Day have to do with boxing?
c.1300, "grass eaten by cattle," from Old French pasture "fodder, grass eaten by cattle" (12c., Modern French pâture), from Late Latin pastura "a feeding, grazing," from Latin pastus, past participle of pascere "to feed, graze" (see pastor). Meaning "land covered with vegetation suitable for grazing" is from early 14c. To be out to pasture "retired" is from 1945, from what was done (ideally) to horses after the active working life.
late 14c., of animals, "to graze;" early 15c., of humans, "to lead to pasture, to feed by putting in a pasture," from Old French pasturer (12c., Modern French pâturer, from pasture (see pasture (n.)). Related: Pastured; pasturing.