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!