"Gentlemen of Uncertain Fortune" by Rory Muir. Fabulous research book!

And yes, as stated elsewhere it is a calendar counting down the 25 days leading up to Christmas.

It's both. 😅 I grew up with a cloth one that had little felt symbols tucked into pockets that you could pull out and reveal each day, but there are one-use ones like a box of chocolates where you open a slot and get one chocolate per day.