> The 25 is there in the name.
This largely depends on who you're asking? I don't know anyone who wouldn't consider 24 the course correct number of Advent, simply because that's the common number here (we celebrate Christmas on the 24th). So 12 makes perfectly sense, just do every second day.
There are also many groups who don't start on the 1st of December, but on the first Sunday of Advent. And probably many others.