This is the name of one of the main adult female characters in Catherynne M. Valente's adult fantasy novel "Palimpsest" (2009). In the novel November remembers having read a book called "The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making" when she was a child, and the heroine of that book was called September. Valente later wrote that book as a crowd-funded work. It became the first volume in her bestselling "Fairyland" series.
It is sometimes claimed to be a feminine form of the Basque masculine name Inar, with the meaning "ray of light", or a feminine name of Arabic origin with the meaning "heaven sent". Both of these origins, however, seem suspicious at best.
In the Middle Ages this name was common in England due to the popularity of medieval tales about the Trojan War. It subsequently became rare, but was revived in the 20th century.