Google Spreadsheet, finding the previous weekday

Question

In Google Spreadsheet, or excel if its the same, how do I find the last weekday of day D only if D is not a weekday itself.

I.E:

If D equals weekday = do nothing (D equals D).
If D equals weekend = D equals last weekday.

Edit: i tried this to get the workday one month away:

WORKDAY(EDATE(N18, 1)+1, -1)

But I Was getting some weird results. Perhaps it counts red days as well, but I never specified the region anywhere.

Edit again:

This above example actually works, eg:

=WORKDAY("2013-04-06"+1, -1)

Answer 1

=WORKDAY("2013-04-06"+1, -1)

...does the trick.