The Yuriban calendar is the primary system of dating used by native Yuribans and by many sympathetic residents of the island. While the Gregorian calendar has been adopted widely and used to date celebrations and festivals as a nod to the majority population of foreigners on the island, the native calendar is the only calendar observed among the peoples of Underhill.
The calendar is lunisolar, incorporating elements of both lunar and solar calendars.
According to the native calendar, the current year (as of 2017 July) is 6840.
In effect the Yuriban calendar consists of two calendars compressed together and synchronized to some extent. The solar year is traced primarily as a religious observance and is of great spiritual importance, while the lunar calendar exists as a practical method of dating day to day and week to week, though it still has many spiritual undertones.
The Yuriban new year begins on the day of the Spring Equinox. Native faith holds that this day is sacred to Harumeku, who moves into primacy over the cycle of seasons and brings life back to the world. This makes the Yuriban year about the same length as the Gregorian year, with some variation depending on when the next equinox falls. The solar year is divided into four quarters by the equinoxes and solstices, all of which are celebrated as important dates.
The cycle of the moon dictates the course of months. The average Yuriban lunar year has twelve months along the lunar cycle, varying between 29 and 30 days. This creates an issue of synchronicity with the solar year, as the lunar cycle of months adds up to only 354 days, creating an 11-day shortfall. The calendar accounts for this by inserting a thirteenth month - a "leap moon" - into the calendar every three years or so. By precedent, in a cycle of nineteen solar years, there will be seven leap months; this follows the Metonic cycle, known in Yuriba since antiquity. That the Metonic cycle is slightly inaccurate has also been known for some time, although the exact value of the inaccuracy was not known until the arrival of outsiders; adjustments have been made manually every few centuries, by removing the second day from a leap month.
Given the lack of perfect synchronicity between the dating of months and years, in some years the first month of the new lunar year will begin several days before the new solar year. In these cases, the month name is prefixed with 'Second' for the duration of the old solar year. The 'Second' is dropped when the new year begins.
Governance of the calendar
The Yuriban calendar is considered to be the province of the priestesses of both Kikoutei and Tsuki, primarily the latter. There is said to be a shrine somewhere in Underhill whereby an appointed priestess from each following meticulously records the passage of time. The duties of naming the months, and of declaring leap months and lost days, falls to the high priestess of Tsuki — though this is largely a formality, given that the same month names and cycles have been used for thousands of years.
The Spring Equinox marks the beginning of spring and of the Yuriban new year. It typically falls around March 20th or 21st on the Gregorian calendar. Other important dates are the Summer Solstice, Autumnal Equinox, and Winter Solstice, which demarcate the seasons and the cycle of seasonal primacy from Harumeku to Zansho to Akibimi to Fuyuzora and back again. Each day is considered sacred to the goddess associated with the coming season.
The other goddesses have sacred days scattered throughout the calendar.
Months of the Year
(Leap Month) Fuillidatu (Fuel-ee-da-tu)