The Caelian Calendar is the calendar used in Orden. A version of it is used throughout the world, with each region taking the format and giving it their own names for the months, holidays, and days of the week.
It has thirteen months, eleven with 28 days and one with 29, summing to 365 days. The calendar was shown and left in the description of "Time and Calendars, Running the Game #34"
The 28 days are divided into 4 seven-day weeks, with each day of the week having a name.
The first 11 months each have 28 days, with the final month having 29. The 29th being a special day to celebrate the years end. While this day is a part of the month, it does not lie in one of the days of the week, so that there is no shift between the day of the week and the day of the month.