The 15th of August can occur on any of the 7 days of the week, so it depends on which year you may be referring to. In 2012, when the question was asked, the last 15th of August, in 2011 was a Monday.
August 15 2002 falls on a Thursday.
August 15 1987 was a Saturday.
15 August 1965 was a Sunday.
15 August 1990 was a Wednesday.
15 August 1970 was a Saturday.
15 August 1947 was a Friday.
15 August 1948 was a Sunday.
15 August 1978 was a Tuesday.
15 August 1942 was a Saturday.
August 15 1947 was a Friday.
August 15 1987 was a Saturday.
August 15 1984 was a Wednesday.