Friday.
You can find a day of week calculator at this site. Just enter in the date and it will calculate it for you:
http://www.searchforancestors.com/utility/dayofweek.html
From a perpetual US calendar: July 23, 1986 was on Wednesday,
August 18, 1986 fell on a Monday.
It was a Wednesday.
Thursday
18 September 1986 was a Thursday.
May 18, 1986 fell on a Sunday.
18 August 1958 was a Monday.
August 18 1946 was on a Sunday.
August 18, 1963, was a Sunday.
August 18 1966 was a Thursday.
August 18 2010 was a Wednesday.
August 18 2000 was a Friday.
August 18, 2009 was a Tuesday.
August 18, 1969 was a Monday.
August 18 1962 was a Saturday.
August 18, 1961 fell on a Friday.