Since the end of the Great War, the Modern Calendar, as it's popularly known, is the calendar in use throughout a good portion of the human countries that participated in The Great War.
As the civilized world found itself embroiled in a global fight for it's survival, the need to standardize dates for military planning became imperative, and a new calendar had to be adopted for the purpose. Afterwards, anyone wishing a military contract, or working for a military contractor or supplier had to use it, so the Modern Calendar quickly percolated down through society. Eventually, even the curmudgenly International Guild of Heralds adopted its use for its official records. So by the war's end, the Modern Calendar was the only calendar known to most people in the war effected countries—though this should not be taken to mean that it's now the only calendar, or even the most commonly used in the post war era. The Balcheri, who were busy fighting a civil war during The Great War, still use their own calendar system, as do the Baktiar, Sun Yani and URhtaani, not to mention the Elves.
The organization of months and festivals is shown in the graphic above. The month names are a hodgepodge that originated primarily in the old Kingdom of Cascadia (now the City State of Fernwall) and the venerable Empire of Sudaan, the oldest known surviving human empire on the planet.
The calendar is loosely based on the old elvish/dwarvish calendar, which is a seasonal calendar system synced to Menelon's two moons. Silvana, the larger of the two, has a twenty eight day cycle. Thus, the year was divided up into 12 months of 28 days apiece. Thorian, the smaller moon, has a 91¼ day cycle. So to each collection of three months of 28 days each, a week (7 days) long festival was added, which is outside any calendar month. (This actually totals 91 days per seasonal period, but allowed the creation of a symmetrical system, which was considered a vast improvement over the rather confusing—if ancient and very thorough—elvish/dwarvish calendar). WinterFest is the exception and is 8 days long. The additional day was necessary to complete Menelon's 365 day solar cycle.
The count of years was taken from the elven calendar, which groups eras (or "ages," as the elves refer to them) by equinoctal cycle. So Year 1 of the new modern calendar was set to the beginning of the current equinoctal (CE) precession cycle. Hence the confusion among the masses as to what "CE" means. While most people believe it means "current era," or "common era," those with a classical education know it really means "current equinoctal", though the error is rarely corrected in polite society.
As Menelon has no mass transit system such as rail, it has not yet occurred to anyone that times should be standardized throughout geographical regions. Quite the contrary, in fact. Every town and village maintains it's own local solar time. For travelers who have spent the day on the road to, upon arriving in a new town, pause and set their pocket watches to the time shown on the town clock is a routine part of travel.
Dates written in numerical form correspond to the manner in which we do it here on Earth, and when so doing, the festivals are counted as months. So 6/15/580 corresponds to the fifteenth day of Læmath, 580, the middle of spring; and 5/3/581 is the third day of Spring Fest, in the year 581. Thus, when writing dates numerically, there are sixteen “months,” rather than twelve.
The ever present exception is WinterFest: If you'll notice, at the top of the calendar, WinterFest is split in half. New Years Day occurs in the middle of the festival (literally midnight of the 4th day). So 1/4/80 is New Year's Eve, 580, and 1/1/81, the fifth day of WinterFest, is also New Year's Day, 581.