The 9th of August can occur on any day of the week. As the question was asked in July 2014, it is possible you were referring to August of 2014, in which case the answer is Saturday.
August 9th, 1979 fell on a Thursday.
its on a Monday!
Tuesday
It was a Monday.
Tuesday
Thursday
Thursday
August 9th 1998 was a Sunday.
August 9th 1986 was a Saturday.
August 9, 1959 was Sunday.
9th August 1969 was a Saturday.
August 9th, 2009 fell on a Sunday.