Aarn is a world that has a 396.5 day year with five 79-80 day months in that year. According to the Zenninfal calendar, Aarn's months are further divided into 15 26-ish day "Constellations" or "Stars."
Depending on where one is in Zenninfal, Yessha and Habruk, the week is usually seven days long. The exception is Igakari and other isolated areas of Eastern Zenninfal, where the week is 11 days long, and the days are named after the eleven celestial bodies that wander Aarn's sky.
The Months and Seasons:
As an important astronomical disclaimer, Aarn's moon, Selmona, is actually a gas giant that orbits Aarn's sun, Vau. Aarn, in turn, orbits Selmona, so Aarn's year is actually Selmona's orbital period, and its months are Aarn's orbital period around Selmona.
Aarnians know this - they know that Selmona is a planet, and Aarn is a moon. Because Aarn's sister moons can be plainly seen orbiting Selmona, this knowledge was painfully obvious to even the earliest of Aarn's astronomers. But to make discussing Aarn's sky and culture easier, I call Selmona Aarn's "moon" because it is the primary object in Aarn's sky that waxes and wanes.
According to the Zeninfal calendar, each new year begins on the new moon following the winter solstice, which on average is 70 days after the solstice. The year can be divided into the four typical seasons, but instead of measuring the seasons via the equinoxes and solstices (which are still quite important to Aarn society), the seasons are measured by months.
Aarn's first month is considered spring, as the planet's northern hemisphere warms from winter. Its next two months are considered Summer and Midsummer, the periods of time when there are moderate to warm temperatures. Aarn's fourth month is considered Autumn, and Winter is Aarn's final month.
Importantly, there is no difference on Aarn between a solar month and a lunar month. Due to the peculiarities of Aarn's orbit, the yearly seasons stay in synch with the months, which always begin with a new moon, have a full moon in the middle of the month (when for a week the night is as nearly bright as a cloudy day) and a new moon again at the beginning of the next month. This is mostly what makes combining months with seasons possible.
Spring - 79 days on odd numbered years, and 80 days on even numbered years. The Spring Equinox usually takes place around the 30th of Spring.
Summer - 79 days. The Summer Solstice usually takes place around the 50th of Summer.
Midsummer - 80 days. The Midsummer Equinox usually takes place around the 70th of Midsummer.
Autumn - 79 days. Autumn has no equinox nor solstice.
Winter - 79 days. The Winter Solstice usually takes place around the 10th of Winter.
Luckily for us Earthings, who happen to be used to thirty day months, Aarnians do use a unit of time of similar length - the "Constellations," or "Stars." Each month is divided up into 3 Constellations, roughly corresponding to the stars that Selmona crosses as it moves across Aarn's sky. These monthly constellations are named after objects or people that are culturally significant to the ancient Igans, but of course, there are far more constellations in the sky that are not directly associated with Aarn's calendar.
The Fountain. Based on the legend of a miraculous fountain that provided a town with fresh water during a harsh winter when even the groundwater froze.
Lepis, The Hare. Based on the legend of a runner who was transformed by Venrisha into a rabbit.
The Rosary. According to legend, this constellation was put into the sky by a divine caster who threw her rosary into the heavens. According to conflicting accounts, it was put into the heavens in defiance of the gods, the gods put it there to shame her for her sacrifice, or the gods made it visible to all so she would never lose it again.
The Rod. (Also known as The Scepter.) Some speculate that this constellation was once a fertility symbol in the sky, though modesty has shifted its original representation to one more suitable to polite company. It is now seen more as a sign of authority, and many mistakenly associate it with The Crown.
The Golem. A constellation that seems to be little more than a stick figure, but it has become associated with strength and magic, especially channeling. Other cultures associate this constellation with Giants, the forebears of most civilized races, in those cultures the constellation is only associated with strength, and not magic.
The Sea Turtle. According to an anicent myth, Aarn is borne by a sea turtle that swims the waters around Aarn's moon.
The Tree. This constellation represents a World Tree that was said to support the heavens and connect them to Aarn.
The Cup. A vessel of flowing liquid, often associated with the brewing of ale and spirits.
Aran, The Warrior. A rival to one of Igakari's great ancient Heroes, Surmemnon. Few are sure why this constellation was named after Aran, and not Surmemnon, though some scholars believe that Aran was the original hero of the Surmemnonic epics, and historical revisonism took place some time in Igakari's history.
The Torch. A constellation said to symbolize the dwindling daylight hours, and also the importance of fire to many cultures and civilizations.
The Mill. A constellation whose origin can be traced back to satyrs, this constellation resembles a windmill, and is associated with strength, ingenuity and providing food for one's family.
Kyrie, The Bird. A harbinger of peace, also known for protecting wanderers and dreamers.
The Silent King's Crown, also known as Rosenius' Crown, or more simply, just "The Crown." King Rosenius was an ancient, overweight ruler many consider to be fictional, but who may have been based on a real person. He is famous for taking a vow of silence after a great calamity befell his people, and then giving away much of his personal wealth.
The Wheel. A nearly circular constellation that resembles a spoked wheel. This constellation is associated with luck and fortune, both good and bad.
The Reaper. A cloaked figure, considered to be a servant of the One Mother (and her equivalents) who collects souls for the afterlife. He is considered a harbinger of the end of the year - and the start of the new.
The Days of the Week:
The areas conquered by the Jerol Empire (Eastern Zenninfal, Habruk, and Yeshha) share the same days of the week, separate from Igakari, which has an eleven day week (with the days named after the visible wandering celestial bodies in the Igan sky). When Habruk conqured the first Jerol tribes, the Jerolans were forced to give up their gods for Habrukan gods. They were allowed to keep their traditional seven day week, but the names of their traditional Ez gods were forcefully replaced by Habrukan gods. Since then, both Habruk and Yessha were conquered by the Jerol Empire, and the Jerol days of the week became canon for half of the world.
Onysday, the first day of the week, and day of renewal. Named after Onys, now known as the One Mother in Western Zenninfal, and Onyrsa in Habruk.
Junday, the day of productivity. Junhin was one of the few ethnic gods the Jerolans were allowed to keep, and as such, the invading Habrukan armies forced their Jerolan captives to work hard on the day of their god, in return for being allowed to keep him. Junhin would eventually become the modern Jennin, molded heavily by that forced work-ethic.
Stosday, the day of purification. This day was the day to cast out all influence from the god Stos, now known as Shtos in Habruk, and Nektos in Western Zenninfal.
Kensday, the day of power, and named after Kenkaran. Strangely, Kenkaran's name is still the same in Habruk today as it was three thousand years ago. Kenkaran eventually became Kennerin in Western Zenninfal.
Dirvsday, the day of reflection, named after Dirvas, the Habrukan Sky Father, god of rationality. Like Kenkaran, his name has changed little in three thousand years. Dirvas, a male, eventually became Western Zenninfal's Divianna, a female.
Fellsday, the day of pennance, named after the ancient god Fellis, who is now known as Fellys in Habruk, and Fagellis in Western Zenninfal. This was the day that all Jerolans were forced to participate in Habrukan religious ceremonies against their will. Themes of sin and self-hatred instilled in them by their Habrukan masters persist to this day.
Korsday, the day of celebration, named after the god Koris, who is now known as Korsan in Habruk, and Coralis in Western Zenninfal. Korsday was traditionally the day of feasting and rest, the one day the Jerolan tribe members were allowed to refresh themselves for the coming work week.
As a final astronomical side-note, an eclipse of the sun by Selmona lasts for more than 8 hours, with a total eclipse lasting for about half of that, far longer than the total eclipse of the sun by the moon on Earth. This length is due to the fact that from Aarn's perspective, Selmona is apparently larger than their sun. Selmona is so large that Vau's corona is usually not visible during the eclipse, though it is sometimes visible during the partial eclipse. Instead, what makes the eclipse spectacular is the huge black sphere in the sky, with Selmona's rings brightly illuminated on either side of it. Eclipse season happens at about twice a year, currently in late Spring and early Autumn, when Aarn's orbit intersects Selmona's shadow. The habitable side of Aarn is not always facing Selmona and the sun during that time.