The Collaborative International Dictionary of English v.0.48:

Week-end \Week"-end"\, n.
   The end of the week, usually comprising the period from
   Friday evening to Monday morning, observed commonly as a
   period of respite from work or school; as, to visit one for a
   week-end; also, a house party during a week-end. Contrasted
   to work days.

   Note: Where work days continue throught Saturday morning, the
         weekend starts on Saturday at noon.
         [Webster 1913 Suppl. +PJC]