The Sendon Calendar was first formalized in 600 C.D. by Emperor Jimmu, who sought a standardized measurement of the seasons' passage to be shared across his united empire. He aimed to ensure unity and consistency among his subjects. Scholars collaborating to fulfill the Emperor's wishes decided on the following: a year would comprise 1080 days, divided into 15 recognized months, with a nine-day week.

The nine days comprising the week were designated as follows: Taidian, Osadian, Ryudian, Kamdian, Yundian, Tordian, Fendian, Joodian, and Tsudian. Originally, the week was planned to begin on Osadian and end on Joodian, in honor of the Emperor and Empress. However, Emperor Jimmu decreed that the week should instead begin on Taidian and end on Tsudian, symbolizing the eternal nature of the heavens, which would endure long after the imperial reign. The remaining days of the week were dedicated in reverence to the sacred beasts, revered for their role in overseeing Sendon and maintaining the harmony of the seasons.

This reverence was further reflected in the 15-month calendar, designed to honor the sacred beasts. Each of the five recognized seasons - Samuon, Tsuyuon, Sakuon, Atsuon, and Kawaon - was allocated three dedicated months, ensuring harmony not only among the elements but also among the sacred beasts. This deliberate balance prevented any beast from feeling slighted or superior to the others.

Samuon

Ginose is the the first month of the Sendon calendar. It contains seventy two days in a month. It's birth flower is Snapdragons, and its gemstone is Kyanite.

Kamose is the second month of the Sendon calendar. It contains sixity five days in a month.It's birth flower is Scillia, and its gemstone is Sapphire.

Cinnose is the last month of the Sendon calendar. It contains eighty five days in a month.It's birth flower is Snowdrop, and its gemstone is Zircon.

Tsuyuon

Kudvia is the third month of the Sendon calendar. It contains seventy two days.It's birth flower is Carnation, and its gemstone is Tourmaline.

Glovia is the fourth month of the Sendon calendar. It contains sixty five days.It's birth flower is Songbells, and its gemstone is Peridot.

Spivia is the fifith month of the Sendon calendar. It contains seventy five days.It's birth flower is Orchid, and its gemstone is Emerald.

Sakuon

Ajisal is the sixth month of the Sendon calendar. It contains eighty five days.It's birth flower is Ajisai, and its gemstone is Charoite.

Sakual is the seventh month of Sendon calendar. It contains sixty five days.It's birth flower is Sakura, and its gemstone is Rose Quartz.

Wistal is the eighth month of Sendon calendar. It contains seventy five days.It's birth flower is Fuji, and its gemstone is Amythest.

Atsuon

Lauridor is the ninth month of the Sendon calendar. It contains seventy two days.It's birth flower is Yellow Tulip, and its gemstone is Citirine.

Peonidor is the tenth month of the Sendon calendar. It contains sixty five days.It's birth flower is Yellow Peony, and its gemstone is Opal.

Quinidor is the eleventh month of the Sendon calendar. It contains seventy five days.It's birth flower is White Narcissus, and its gemstone is Amber.

Kawaon

Auraire is the twelfth month of the Sendon calendar. It contains seventy two days.It's birth flower is Red Poppy, and its gemstone is Red Spiniel.

Cypaire is the thirteenth month of the Sendon calendar. It contains sixty five days.It's birth flower is Red Rose, and its gemstone is Ruby.

Zelaire is the fourteenth month of the Sendon calendar. It contains seventy two days.It's birth flower is Red Hibiscus, and its gemstone is Garnet.

Each month also has two potential star signs they are connected to but see Astrology & Astronomy for more details.