The word “Boxing Day” doesn’t exist in French. Instead, we might say:
le lendemain de Noël /luh lond maind no elle/.
But there is, however, a word for the day after an intoxicated evening:
le lendemain de veille /luh lond maind vay yuh/.
After a lendemain de veille, it is not uncommon to have la gueule de bois /lah guhl duh bwa/ (hangover). Literally: having, getting the “wooden mug”.
Hoping this is not your case today, I wish you a happy Boxing Day!