The Collaborative International Dictionary of English v.0.48:

Workday \Work"day`\, n. & a. [AS. weorcd[ae]g.]
   A day on which work is performed, as distinguished from
   Sunday, festivals, etc., a working day.
   [1913 Webster]

WordNet (r) 3.0 (2006):

workday
    n 1: a day on which work is done [syn: workday, working day,
         work day] [ant: day of rest, rest day]
    2: the amount of time that a worker must work for an agreed
       daily wage; "they work an 8-hour day" [syn: workday,
       working day]