Cut them in half, cut 12 colored index cards slightly taller than the date cards to act as separations between the months.
Mod-podged them to the boxes.
Here's the stack of cards. Clearly, the whole box would be too big for this little stack of paper cards. I didn't want them to slump over, but I also want the box to remain this size so that if ticket stubs or other paper ephemera turns the stack fatter over the years, it will have room to grow.
Solution? Cut bits of foam to size, so that the box is the size I want it to be now. If the user of the box wants more room in the future, they can easily cut the foam down to size again. I then cut a piece of contrasting origami paper to fit over the foam so that it's not so unsightly. First I tucked it under the stack of foam (I would have used straight pins to keep it taught if I'd had any).
And tuck it in, behind the foam.
Now, every day for the next 14 years, I write something about that day on one line of the index card. The calendar will be more and more fun to read as the years go by. It's an exercise in brevity and patience.