If 6th March 2005 was a Monday, then on which day of the week did 6th March 2004 fall?

Introduction / Context:
This calendar problem links the same date in two consecutive years and asks you to work backwards from a known weekday. It also involves a leap year, so you must understand how leap years change the weekday shift compared to ordinary years.

Given Data / Assumptions:

• 6th March 2005 was a Monday (as stated in the question).
• We must find the day of the week on 6th March 2004.
• 2004 is a leap year (since it is divisible by 4 and not a century year).
• We use the usual rule: a leap year contributes 2 odd days, and an ordinary year contributes 1 odd day.

Concept / Approach:
When moving from a leap year to the next year or vice versa, the number of odd days between corresponding dates helps determine the weekday shift:

• From a non-leap year to the next year on the same date: weekday moves forward by 1 day.
• From a leap year to the next year on the same date (for dates on or after 1st March): weekday moves forward by 2 days because of the extra leap day.

In this problem, 2004 is a leap year, and 2005 is the following ordinary year, so from 6th March 2004 to 6th March 2005 the weekday advances by 2 days. We are given the later date's weekday and must move backward.

Step-by-Step Solution:
Step 1: Note that 2004 is a leap year, so the interval from 6th March 2004 to 6th March 2005 spans one leap year. Step 2: For dates on or after 1st March in a leap year, the same date in the next year will be 2 weekdays ahead. Step 3: According to the question, 6th March 2005 is a Monday. Step 4: That means 6th March 2005 is 2 days ahead of 6th March 2004. Step 5: To find 6th March 2004, move 2 days backward from Monday. Step 6: One day back from Monday is Sunday, and two days back is Saturday. Step 7: Therefore, 6th March 2004 was a Saturday.

Verification / Alternative check:
You can verify the logic by assuming the 2004 date first. If 6th March 2004 is Saturday, moving forward 2 days (because 2004 is a leap year) gives Monday for 6th March 2005, which matches the question's statement. This reverse-and-forward consistency check confirms that our reasoning is correct.

Why Other Options Are Wrong:
Sunday: This is only 1 day behind Monday, whereas we require a 2-day backward shift.

Monday: This implies no shift at all, which contradicts the leap-year effect between 2004 and 2005.

Tuesday: This is 1 day ahead of Monday and would be incorrect when working backward in time.

Friday: This is 3 days behind Monday, which does not align with the exact 2-day shift needed.

Common Pitfalls:
A typical mistake is to forget the difference between dates before and after 1st March in a leap year. For dates before 29th February, the shift behaviour differs. Another frequent error is to move in the wrong direction in time or to assume a 1-day shift instead of 2 days for leap years. Always remember that for dates on or after 1st March of a leap year, the same date next year is 2 weekdays ahead, and working backward reverses this by 2 days.

Final Answer:
6th March 2004 fell on a Saturday.