I created this calendar in the Visual Communication class at the university. I decided to literally square the circle - each month's figure is created from basic division of a square in the circle. Take a look at the explanatory diagram below.
I wanted to keep it as simple as possible. I used only 3 colors (black&white, pink) and 3 different textures (plain, polka dots, stripes in various orientations), which resulted in 14 different styles.