Why is the ninth month called September?
mid-14c., from Old French satin (14c.), perhaps from Arabic (atlas) zaytuni, literally "(satin) from Zaitun," a Chinese city, perhaps modern Quanzhou in Fukien province, southern China, a major port in the Middle Ages, with a resident community of European traders. The form of the word perhaps influenced in French by Latin seta "silk." OED finds the Arabic connection etymologically untenable and takes the French word straight from Latin. As an adjective from mid-15c.