Why is the ninth month called September?
"ordinary" (1580s); "insignificant" (1590s), from Latin trivialis "common, commonplace, vulgar," literally "of or belonging to the crossroads," from trivium "place where three roads meet," in transferred use, "an open place, a public place," from tri- "three" (see three) + via "road" (see via). The sense connection is "public," hence "common, commonplace."
The earliest use of the word in English was early 15c., a separate borrowing in the academic sense "of the trivium" (the first three liberal arts); from a Medieval Latin use of trivialis in the sense "of the trivium," from trivium as neuter of the Latin adjective trivius "of three roads." Cf. trivia. Related: Trivially. The board game Trivial Pursuit was released 1982 and was a craze in U.S. for several years thereafter.