Why is the ninth month called September?
mid-13c., "rich cloth" (often, but not necessarily, bright red), from a shortened form of Old French escarlate "scarlet (color), top-quality fabric" (12c., Modern French écarlate), from Medieval Latin scarlatum "scarlet, cloth of scarlet" (also source of Italian scarlatto, Spanish escarlate), probably via a Middle Eastern source (cf. Arabic siqillat "fine cloth"), from Medieval Greek and ultimately from Late Latin sigillatus "clothes and cloth decorated with small symbols or figures," literally "sealed," past participle of sigillare, from the root of sign (n.).
In English as the name of a color, attested from late 14c. As an adjective from c.1300. Scarlet lady, etc. (Isa. i:18, Rev. xvii:1-5) is from notion of "red with shame or indignation." Scarlet fever is from 1670s, so called for its characteristic rash. Scarlet oak, a New World tree, attested from 1590s. Scarlet letter traces to Hawthorne's story (1850). German Scharlach, Dutch scharlaken show influence of words cognate with English lake (n.2).
This dye was obtained by the Egyptians from the shell-fish Carthamus tinctorius; and by the Hebrews from the Coccus ilicis, an insect which infests oak trees, called kermes by the Arabians. This colour was early known (Gen. 38:28). It was one of the colours of the ephod (Ex. 28:6), the girdle (8), and the breastplate (15) of the high priest. It is also mentioned in various other connections (Josh. 2:18; 2 Sam. 1:24; Lam. 4:5; Nahum 2:3). A scarlet robe was in mockery placed on our Lord (Matt. 27:28; Luke 23:11). "Sins as scarlet" (Isa. 1:18), i.e., as scarlet robes "glaring and habitual." Scarlet and crimson were the firmest of dyes, and thus not easily washed out.