From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
Monday
n 1: the second day of the week; the first working day [syn:
Monday, Mon]