What does Boxing Day have to do with boxing?
1550s, "of or at the vertex, directly overhead," from Middle French vertical (1540s), from Late Latin verticalis "overhead," from Latin vertex (genitive verticis) "highest point" (see vertex). Meaning "straight up and down" is first recorded 1704.
vertical ver·ti·cal (vûr'tĭ-kəl)
Of or relating to the vertex of the head.
Being or situated at right angles to the horizon; upright.