The Collaborative International Dictionary of English v.0.48:

Thursday \Thurs"day\, n. [OE. [thorn]ursdei, [thorn]orsday, from
   the Scand. name Thor + E. day. Icel. [thorn][=o]rr Thor, the
   god of thunder, is akin to AS. [thorn]unor thunder; D.
   Donderdag Thursday, G. Donnerstag, Icel. [thorn][=o]rsdagr,
   Sw. & Dan. Torsdag. [root]52. See Thor, Thunder, and
   Day.]
   The fifth day of the week, following Wednesday and preceding
   Friday.
   [1913 Webster]

   Holy Thursday. See under Holy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Thursday
    n 1: the fifth day of the week; the fourth working day [syn:
         Thursday, Th]