The very center of the calendar is Sol-- our sun. Around it are the orbits of the planets: Mercury, Venus, Earth, Mars, Jupiter, Uranus, Neptune. Each is represented by its Latin symbol and by its symbolic color.
As you know, the Earth spins around the sun. One spin around the sun is a year. This is the basis of the calendar's design! The calendar precisely represents the Earth's cycle around the sun in one year-- with that circle being divided into 365 days.
Joining the Earth in its yearly spin are the seven other planets that make up our solar system. In the center, the Earth's orbit is blue. It is a full thick line. You can see the orbits of the other planet's around the Earth. It's circle is colored corresponding to its planet's color (see the lengend) and marked with its roman symbol.
All the other planet's have different orbital periods. The inner planets spin more than once around the sun in 365 days. The outer planets spin less than once. The full colored circle of the planet represents its beginning position at the start of the year. The solid circle represents its position at the end of the year.
The thick line represents the amount of it's orbit that it has completed in the year.
The Calendar also shows net distance of each planet as it increases and decreases over the year to the Earth. This can be handy to know a planet is approaching opposition and when it will be brightest in the sky. When the planet is furthest from the Earth it will be at the extremity of the Calendar's day zone. When the planet is closest to the Earth, it will be at the bottom of the day zone.