Why is the ninth month called September?
Old English læg, leag "lye," from Proto-Germanic *laugo (cf. Middle Dutch loghe, Dutch loog, Old High German louga, German Lauge "lye"), from PIE root *leue- "to wash" (see lave). The substance was formerly used in place of soap, hence Old High German luhhen "to wash," Old Norse laug "hot bath, hot spring," Danish lørdag, Swedish lördag "Saturday," literally "washing-day." Chamber-lye in early Modern English was the name for urine used as a detergent.