the day after a holiday
Refers to the first working or school day after a holiday or break, often with a connotation of returning to routine.
Mondays after a holiday are always busy.
It's the day after a holiday, and I still feel sluggish.
Broader term for the day after any break (vacation, leave, etc.), not limited to public holidays.
Compound of 休日 (holiday) + 明け (opening, beginning after). The exact historical derivation is straightforward; the word follows a common pattern for 'the day after X'.