What does Boxing Day have to do with boxing?
early 14c., "lose brightness, grow pale," from Old French fader "become weak, wilt, wither," from adj. fade "pale, weak, insipid" (12c.), probably from Vulgar Latin *fatidus, some sort of blending of Latin fatuus "silly, tasteless" + vapidus "flat, flavorless." Related: Faded; fading. As a noun, from c.1300.
To depart, esp gradually (1820+)