Mon·day 
(mŭndē, -dā′)
(mŭndē, -dā′)n. Abbr. Mon. or M
The day of the week that comes after Sunday and before Tuesday. In many countries and according to international standards, Monday is considered the first day of the week.
[Middle English, from Old English Mōnandæg (translation of Latin lūnae diēs, day of the moon) : mōnan, genitive of mōna, moon; see MOON + dæg, day; see DAY.]