The Errindian Calendar

The calendar of Errindia is composed of 8 months and 4 intercalary days. Each month is 25 days, or 5 weeks of 5 days each. An intercalary day falls between every other month. In total, a year has 204 days.
The year begins on the first day of spring, Hiron 1st. There is one day between Hiron and Rivend: the spring equinox, marking the height of spring. The summer solstice falls between Arkris and Thrindara, the autumnal equinox falls between Chrahn and Chydeth, and the winter solstice falls between Festeloh and Gaitrune. Each of these days is marked by a holiday observed throughout most of the world: Birthright, Tribute, Remembrance, and Candlewatch respectively.
The moon has a 51 day cycle, with the full moon always occurring on the solstices and equinoxes. Thus, the moon waxes and wanes with each season, beginning new on the first days of Hiron, Arkris, Chrahn, and Festeloh.
The 5 days of the week are named for the story of the founding of the Maldetian Empire: Maldet for the hero who won the hearts of his people, Ghandt for the beast he slayed to do so, Viper for the sword with which he slew it, Jölan for his wife who set the empire's sights on the world, and Errin for his daughter who saw her parents' dream come to fruition. Under its control, the world came to use the calendar set out by the Empire, and much of the world still follows it in the Empire's absence.
The months are each named for a constellation visible from Ra Einshar during that month, though the names gradually changed as people speaking different languages around the world adjusted to a Common tongue.