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Note that the four godly months were full months of 31 days, with the remainder hollow and only 30 days. Why question are frequently impossible to answer, with reasons lost long past in antiquity. We are left to surmise that the naming scheme was chosen to reflect the length difference.
Yes, the stylistic differences are the result of the two forms being of different declension.
Here is a representative declension of the Latin noun September:
and for comparison Junius:
For the uninitiated, a short primer on Latin noun declensions with English equivalents:
Upvote:4
I couldn't find the source but once I read that the five gods naming the months were the most important gods in the Ancient Roman religion, before it received Celtic and Greek influences:
March - The first month named for a Father God, Mars. Mars was the most important god for the original Romans, because Romulus and Remus were their sons. And Jupiter means etymologically "Father Sky" (like Zeus means "God").
April - For a goddess of Love, Aphrodite.
May - For an Earth goddess, Maia, the ubiquos agricultural deity in all the Mediterranean ancient socities.
June - For a Mother Goddess, Juno.
So the four main aspects: father, mother, love and agriculture. Without marriages and children, a society don't last.
January was named for Janus, an Old God akin to Chaos.
January was named after Janus, a sky-god who was ancient even at the time of Romeβs founding. Ovid quoted Janus as saying "The ancients called me chaos, for a being from of old am I." After describing the worldβs creation, he again quoted Janus: "It was then that I, till that time a mere ball, a shapeless lump, assumed the face and members of a god." http://www.webexhibits.org/calendars/calendar-roman.html
And February was probably named because it was the last month of the year and its name meant "purification", so they could expiate before starting the new year.