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.
Thursday
Thursday
Tuesday
It was a Monday.
its on a Monday!
Tuesday
August 9, 1959 was Sunday.
August 9th 1986 was a Saturday.
August 9th 1998 was a Sunday.
August 9th, 2010 fell on a Monday.
August 9th, 2009 fell on a Sunday.