What does Boxing Day have to do with boxing?
late 14c., "periodical sitting of a court," from Old French session "act or state of sitting; assembly," from Latin sessionem (nominative sessio) "act of sitting; a seat; loitering; a session," noun of action from past participle stem of sedere "to sit" (see sedentary). Sense of "period set aside for some activity" is first recorded 1920, in bull session, probably from quarter sessions courts (see quarter (n.)). Musical sense of "recording occasion in a studio" is from 1927.
: He used to do a lot of session playing but hardly ever worked for an audiencenoun