Do you care about holidays or just saturday / sunday?

Do you have access to (or are you willing to create and maintain) a calendar table?

If you need to discount holidays, you need a calendar table. It would likely be a good idea if you don't care about holidays

If you don't care about holidays, and don't have or want a calendar table, the logic you need to determine if any date is the 5th business day of the month is like this:

CASE WHEN DATEPART(weekday,[Date]) = 6 AND DATEPART(day,[Date]) IN (5,6) THEN 1

WHEN DATEPART(weekday,[Date]) IN (2,3,4,5) AND DATEPART(day,[Date]) = 7 THEN 1

ELSE 0 END

Where [Date] is your date field, giving you a 1 when that date is the 5th business day of the month, and a 0 otherwise.

If you want to use this in a query though, you might want to give us some sample ddl, data, and expected output, because throwing the above into a where clause might perform terribly.