It has often been on a Monday. You are probably referring to the year 2001, in which it was on a Tuesday.
September 11th, 2028 will fall on a Monday.
September 11th 1995 was a Monday.
September 11th, 1972 fell on a Monday.
September 11th, 1899 fell on a Monday.
The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.The 11th of September in 2001 was on a Tuesday.
July 11th, 2011 fell on a Monday.
11th December 1978 was a Monday.
March 11th, 1963 fell on a Monday.
In 2010, the second Monday in October is the 11th.
It was a Monday.
It was a Monday.
It was a Monday.