Why is the ninth month called September?
c.1400, "dark," figuratively "morally unenlightened; gloomy," from Old French obscur, oscur "dark, clouded, gloomy; dim, not clear" (12c.) and directly from Latin obscurus "dark, dusky, shady," figuratively "unknown; unintelligible; hard to discern; from insignificant ancestors," from ob "over" (see ob-) + -scurus "covered," from PIE *(s)keu- "to cover, conceal" (see sky). Related: Obscurely.
early 15c., "to cover (something), cloud over," from obscure (adj.) or else from Middle French obscurer, from Latin obscurare "to make dark, darken, obscure," from obscurus. Related: Obscured; obscuring.
"A Formal Description of the Specification Language OBSCURE", J. Loeckx, TR A85/15, U Saarlandes, Saarbrucken, 1985.