Wizard's Reckoning and the Wheel of the Year
In the world of Prydein and the United Kingdoms, Druids lead the people and long ago established a calendar based on two halves of the year, a light half and a dark half. Each half has two seasons, and each season five Callans. Callans are named after trees with particular magical properties and correspond also to animals, herbs, stones and many other parts of nature. Each Callan has 18 days divided into three six-day weeks (there's no Saturday).
The diagram below is a small version of a more legible PDF you can read or download here.