Why is the ninth month called September?
late 13c., probably from Old English *læððe, variant of lætt "lath," apparently from a Proto-Germanic *laþþo (cf. Old Saxon, Old Norse latta, Middle Dutch, German latte "lath," Dutch lat, Middle High German lade "plank," which is source of German Laden "counter," hence, "shop"). As a verb, 1530s, from the noun.
any material fastened to the structural members of a building to provide a base for plaster. Lath can be of wood, metal, gypsum, or insulated board. In older residential buildings, narrow wood strips were generally used.