Mon·day / ˈməndā/ • n. the day of the week before Tuesday and following Sunday: I saw him on Monday the Monday before last | [as adj.] Monday morning. • adv. on Monday: I'll call you Monday. ∎  (Mondays) on Mondays; each Monday: the restaurant is closed Mondays.

Monday the day of the week before Tuesday and following Sunday. Recorded from Old English (in form Mōnandæg), the name originally meant ‘day of the moon’, a translation of late Latin lunae dies.
Monday club a right-wing political club, formed in 1961, that originally held its meetings on Mondays.
Monday morning quarterback in North America, a person who passes judgement on and criticizes something after the event, an armchair critic.
Saint Monday used with reference to the practice among workmen of being idle on Monday, as a consequence of drunkenness on the Sunday, chiefly in the phrase to keep Saint Monday.

Monday OE. mōnandæġ, corr. to MLG., MDu. mān(en)dach (Du. maandag), OHG. mānatag (G. Montag), ON. mánadagr; f. MOON + DAY, tr. late L. lūnæ diēs ‘day of the moon’ (after Gr. hēmérā Selénēs).