If 6th March, 2005 is Monday, what was the day of the week on 6th March, 2004?

The year 2004 is a leap year. So, it has 2 odd days.

But, Feb 2004 not included because we are calculating from March 2004 to March 2005. So it has 1 odd day only.

 The day on 6^th March, 2005 will be 1 day beyond the day on 6^th March, 2004.

Given that, 6^th March, 2005 is Monday.

 6^th March, 2004 is Sunday (1 day before to 6^th March, 2005).