The Order of Hermes divides the year into 4 Seasons - Spring, Summber, Autumn and Winter. These correspond to the medieval year, though in Europe the start date of each year can vary depending on which country you are in.
As far as the Order is concerned, the start of each Season is based on the Solstices and Equinoxes:
In much of Europe, the Church considers new year to begin sometime around March/April. All dates in this campaign will use the Order of Hermes reckoning however, for simplicity.