I'm a bit of a boffin, sorry, when it comes to the origins of Advent Calendars as we did a short study on them during my German degree. (In Buddenbrooks, the renowned novelist Thomas Mann writes about a homemade tear-off calendar which the nanny makes for the child. That was in 1901 if I remember.)
Anyway, the first advent calendars on paper originated from Germany and had texts, sometimes religious, sometimes not, for every day leading to Christmas. I haven't time at the moment to find the names, I'm on child duty, but there were companies manufactuing them in 1930s when businesses started.
During World War 11 it was verboten to produce calendars with pictures. Instead, the Nazis chillingly produced their own version of an Advent Calendar which consisted of a pamphlet with images with swastikas and tanks being blown up.
I think it was the Americans in the 1950s who took over the idea of Advent Calendars in a big way, introducing chelated and sweets. Then the idea of gifts grew from there.