What does Boxing Day have to do with boxing?
"to insert a day into the calendar," 1610s, from Latin intercalatus, past participle of intercalare "to proclaim the insertion of an intercalary day," from inter- "between" (see inter-) + calare (see calendar). Related: Intercalated; intercalating.
intercalated in·ter·ca·lat·ed (ĭn-tûr'kə-lā'tĭd)adj. Inserted between two others; interposed.