The Collaborative International Dictionary of English v.0.48:

Monday \Mon"day\ (m[u^]n"d[asl]; 48), n. [OE. moneday, monenday,
   AS. m[=o]nandaeg, i.e., day of the moon, day sacred to the
   moon; akin to D. maandag, G. montag, OHG. m[=a]natag, Icel.
   m[=a]nadagr, Dan. mandag, Sw. m[*a]ndag. See Moon, and
   Day.]
   The second day of the week; the day following Sunday.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Monday
    n 1: the second day of the week; the first working day [syn:
         Monday, Mon]

The Devil's Dictionary (1881-1906):

MONDAY, n.  In Christian countries, the day after the baseball game.