Monday
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.
15th August 1949 was on a Monday.
August 15th 1977 was a Monday.
15th August 1947 was a Friday.
15th August 1952 was a Friday.
August 15th 1960 was a Monday.
August 15th 1996 was a Thursday.
August 15th 1929 was a Thursday.
15th, August, 2000, was a Tuesday.
Joe Jonas' Birthday is august 15th. August 15th is my b-day!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!AHHHHHHHHH!!!!!!!!!!!!
15th August 1947 was a Friday.
August 15th 1972 was a Tuesday.