What does Boxing Day have to do with boxing?
1530s, "secret, not divulged," from Middle French occulte and directly from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from ob "over" (see ob-) + a verb related to celare "to hide," from PIE root *kel- "to hide" (see cell). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1540s. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1630s.
occult oc·cult (ə-kŭlt', ŏk'ŭlt')
Detectable only by microscopic examination or chemical analysis.
Not accompanied by readily detectable signs or symptoms.