Sunday.
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
September, 12th, 1956, was a Wednesday.
September 13, 1960 fell on a Tuesday.
December 29 1956 was a Saturday.
September 12, 1959 fell on a Saturday.
August 13, 1961 fell on a Sunday.
Lassie - 1954 was released on: USA: 12 September 1954 West Germany: 8 February 1960 Netherlands: 6 October 1962 France: 13 September 1972
The Jayhawkers - 1959 was released on: USA: 15 October 1959 West Germany: 24 June 1960 Sweden: 25 July 1960 Finland: 19 August 1960 Mexico: 25 August 1960 Norway: 21 November 1960 France: 13 January 1961 Denmark: 4 September 1961
When Comedy Was King - 1960 was released on: USA: 29 March 1960 Finland: 13 May 1960 Mexico: 13 August 1960 Sweden: 31 October 1960 Austria: 11 November 1960 Denmark: 13 December 1960 Japan: 17 December 1961 East Germany: 11 January 1963
Man from Interpol - 1960 The Murder Racket 1-13 was released on: UK: 1960 USA: 23 April 1960
From a perpetual US calendar: September 13, 1960 was on Tuesday.
September 13 1960 was a Tuesday.
February 13 1960 was on a Saturday.
March 13 1960 was a Sunday.
October 13 1960 was on a Thursday.
July 13 1960 was a Wednesday.
March 13 1960 was a Sunday.
December 13, 1960 fell on a Tuesday.
13 September 1995 was a Wednesday.
September 13 1779 was a Monday.
September 13 1980 was a Saturday.
September 13 1949 was on a Tuesday.