The Collaborative International Dictionary of English v.0.48:

Saturday \Sat"ur*day\ (?; 48), n. [OE. Saterday, AS. Saeterdaeg,
   Saeterndaeg, Saeternesdaeg, literally, Saturn's day, fr. L.
   Saturnus Saturn + AS. daeg day; cf. L. dies Saturni.]
   The seventh or last day of the week; the day following Friday
   and preceding Sunday.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Saturday
    n 1: the seventh and last day of the week; observed as the
         Sabbath by Jews and some Christians [syn: Saturday,
         Sabbatum, Sat]