As a US based longtime reader of Agatha Christie, I must say that was one of the most confusing locutions! I grew up in the 60s and 70s - my reference for boxing was Muhammad Ali.
After two days of take out....I cooked. Lots of work and untold amount of dishes. We used to go DT to Boxing Day sales, but not now. We watched old movies and relaxed (well I cooked all day!). Nice to have leftovers today and I did hit the mall. Madhouse!!! But I got my steps in..... Back to cooking tomorrow.