Step 1: Select colors and stamp
If you don't have stamps, simply use a sharpie =)
Step 2: Print, cut, glue and arrange
Anyway, if you’d like to make such a calendar - I attached a free download for you as a PDF :)
Print it (12 copies of page 1 or, to be correct, 7 copies of page one, 4 copies of page two, and 1 copy of page three) and cut it to the width of your paint swatches. Trim to get an equal height. Choose a nice place for your calendar and attach it to your wall or door.
I always mark the current date with a little arrow (“Book Dart”), so I can see upcoming birthdays at a glance.