working day vs workday

working day and workday both are nouns.

Word	Noun	Adjective	Verb	Adverb
working day	Yes	No	No	No
workday	Yes	No	No	No

As nouns, working day and workday are synonyms defined as:

working day and workday: the amount of time that a worker must work for an agreed daily wage

As nouns, working day and workday are synonyms defined as:

Other synonyms of working day include work day.

working day (noun)	workday (noun)
the amount of time that a worker must work for an agreed daily wage	the amount of time that a worker must work for an agreed daily wage
a day on which work is done	a day on which work is done