You need the two squares of paper inside the cd case: front and rear.

Take the front square and, on the paperboard, mark the exact measure in red colour. That will become the height of the calendar.
Repeat the same thing with measure in blu colour. That will become the width of the calendar.

Next to the first square draw a second square with the same measures.

Then cut out the rectangle and fold it in the center.

Now there's the fun part.
Use a square for each month (for each rectangle of paper there are two months).
Be careful to measure the space you need to write the days of the month and, if you like, you can use different colors for mark the special days to remember birthdays, holidays and anniversaries.
Remember to write the name of the month and year!

At the top of the square (the area where the rectangle is folded) you can use all your creativity.

open the cd case and disconnect the two pieces. Turn down the second piece (the one that "contains" the CD) and fits the two pieces back together.

Don't mind the scotch on my CD Case :)

Lifts the lid of the CD case and insert the month.
The second piece of CD case becomes the basis of the calendar.

Behind every month you can write what happens in the days that you have color on the calendar, for example the name of the person's birthday.

uses a strip of paper to write the "title" of the calendar and apply it inside the base.

If you don't know where to put the squares with the other months of the year you can keep them inside the base of the calendar.