An early 20th century book I bought had a small rectangular page from a tear-off calendar inside like a bookmark: from the font and day/date I deduced it was probably 1910s or 20s . I’ve never removed it or changed the page marked: it felt like a ghostly connection across the decades.

(Can look it out & send details/pics if this would be a help for your research: not sure about the bookshop I got it from though, might have been a store on eBay)

Thank you! Do you remember the title/author of the book?