Monday. It would probably have been quicker and simpler to look at a calendar.
September 6 2010 was a Monday.
Adding 100 days to September 6 in any year gives December 15 in the same year.
Labor Day in 2012 is on Monday, the 3rd of September. In the United States, Labor Day is always celebrated on the first Monday in September
6 September 1972 was a Wednesday.
6 September 1958 was a Saturday.
September 6 1950 was on a Wednesday.
September 6, 1970 was a Sunday.
September 6 1952 was a Saturday.
September 6 1929 was a Friday.
September 6 1963 was a Friday.
September 6, 1955 was a Tuesday.
September 6, 2008 was a Saturday.