The calendar used in Faerûn was invented by the wizard Harptos. He also defined a zodiac, as can be seen in the table below, with the most prominent constellation in the night sky for each month. This zodiac was created for astronomical purposes, but somehow fell into people's liking.

It is a common belief among all the simple folk across Faerûn that people's personality and temperament is dependent of their zodiac signs. As such, they had to fill up the 5 fest days with a sign. Harptos did that himself, but instead of the fest days "borrowing" a constellation from a neighboring month, they have their own signs related to the most brilliant star in the night sky in those days. (Note that people born in the day of the Shieldmeet are considered to be born on the 1st of Eleasis.)