Why is the ninth month called September?
mid-14c., from Old French esclate, fem. of esclat "split piece, splinter" (Modern French éclat; see slat), so called because the rock splits easily into thin plates. As an adjective, 1510s. As a color, first recorded 1813. Sense of "a writing tablet" (made of slate), first recorded late 14c., led to that of "list of preliminary candidates prepared by party managers," first recorded 1842, from notion of being easily altered or erased. Clean slate (1856) is an image from customer accounts chalked up in a tavern.
1520s, "to cover with slates" (earlier sclatten, late 15c.), from slate (n.). Meaning "propose, schedule" is from 1883; earlier "to nominate" (1804); the notion is of writing on a slate board. Related: Slated; slating.