If All Saints’ is observed with an Octave, which I would bet on at the time, then the Monday after the day is another way of saying the Monday within the Octave. If one’s life revolves around the liturgical calendar, this would be a very good way of identifying a day.
Interesting discussion.
I have a Missal published in 1949 which still has November 8 as “Octave-Day of All Saints.” It says “Mass as on the feast” (but the Collect, Secret, and Postcommunion are different).