In America, everyday is a work day! Since the "Blue Laws" (Sabbatical) were eliminated, there is no longer any legal basis to categorize one day from another on the basis of whether it is legal to work (labor) or not. Common usage will separate weekday from weekend, but would not correlate workday to weekday. Workday is what each person experiences on their own. Business day has no common usage in America, might be considered awkward usage, unless as a title to media.

Yes, thank you Ralph!

@Ralph

go to the post office website, USPS.com and look how long it takes to ship a package. I think you'll find interesting usage of the words there :)

I have no clue about legal usage though :(

Wikipedia liefert immerhin eine passende Erklärung zu "Business day":
http://en.wikipedia.org/wiki/Business_day