Ysennian Royal Time

Ysennian Royal Time is the most popular time and date system in use on the material plane of Ysenna.

Calendar
The Ysennian Royal calendar consists of thirteen months, most consisting of 39 or 40 days. The calendar is further divided into eight-day weeks.

Since the Ysennian year is exactly 512 days long, the 8-week cycle repeats every eight years. Also, randomly selecting a day from the year is (relatively) simple, using the following formula:

1d512 = (1d8 - 1) * 64 + (1d8 - 1) * 8 + 1d8

(This is essentially the same operation performed when simulating a 1d100 roll with 2d10, just in base-8. In its native base, the calculation appears much simpler:

1d10008 = 1d108 * 1008 + 1d108 * 108 + 1d108

All dates are equally likely to appear.)

The final day of the year, number 512, is not traditionally considered to be a day of the yearly calendar. Instead, the day is termed [x] or "Day Zero" and is viewed as a sort of hiccup in the yearly cycle. Because of this, asking a native Ysennian the number of days in a year will undoubtedly yield the answer "511".

Months
The thirteen months of the Ysennian Royal calendar are as follows: An additional 4 days appear at the end of the thirteenth month. The first three of these are holidays (see below) and the final is considered a "nonexistent" day.
 * Jessenara (40 days), from Duer medja Jesenara, "Month of Jesena (Ysenna)", the Elder Goddess and figurehead of creation.
 * Navara (39 days) from Duer medja Navara, "Month of Nava (Naa)" the Elder God of Souls.
 * Iska (40 days) named for Ishke Sohep-ateph.
 * [4] (39 days)
 * [5] (40 days)
 * [6] (33 days) [explanation]. The only month of the year not to contain 39 or 40 days, [6] is the shortest month of the Ysennian Royal calendar.
 * Solebar (40 days) from Terran medj-xa Solebarve, "Month of Soleb (Sohep)", the avatar of the god of Fire. Likely named due to its position in midsummer
 * Astrebar (40 days)
 * [9] (39 days)
 * [10] (40 days)
 * [11] (39 days)
 * [12] (40 days)
 * [13] (39 days)

Holidays
pass

Sample Year
Below is a sample calendar for a hypothetical year, including days of the week and popular holidays. This exact arrangement of dates only occurs once every eight years.

(For ease of RNG-date-finding, the total number of days passed is given at the beginning of each month.)

Month 1 begins on day 1. Month 2 begins on day 41 Month 3 begins on day 80. Month 4 begins on day 120. Month 5 begins on day 159. Month 6 begins on day 199. Month 7 begins on day 232. Month 8 begins on day 272. Month 9 begins on day 312. Month 10 begins on day 351. Month 11 begins on day 391. Month 12 begins on day 430. Month 13 begins on day 470.