Source : Webster's Revised Unabridged Dictionary (1913)

Midweek \Mid"week`\, n.
   The middle of the week. Also used adjectively.

Source : WordNet®

midweek
     adj : occurring during the middle of the week; "midweek travel";
           "midweekly prayer meetings" [syn: {midweekly}]

midweek
     adv : in the middle of the week