What does Boxing Day have to do with boxing?
1908 as a political term, a borrowing of French détente "loosening, slackening" (used in the Middle Ages for the catch of a crossbow), from Vulgar Latin detendita, fem. past participle of Latin detendere "loosen, release," from de- "from, away" (see de-) + tendere "stretch" (see tenet). The reference is to a "relaxing" in a political situation. The French word was earlier borrowed as detent (1680s) "catch which regulates the strike in a clock."
A period of lessening tension between two major national powers, or a policy designed to lessen that tension. Détente presupposes that the two powers will continue to disagree but seeks to reduce the occasions of conflict.