If 6th March 2005 is assumed to be a Monday, then, using standard calendar rules, what was the day of the week on 6th March 2004 (i.e., exactly one year earlier)?

Introduction / Context:
This problem tests your understanding of how days of the week shift from one year to the next, especially when leap years are involved. You are given the day of the week for 6th March 2005 and asked to deduce the day of the week on the same date one year earlier, 6th March 2004, using only calendar logic.

Given Data / Assumptions:

• 6th March 2005 is given as a Monday (assumed for the question).
• We want the weekday on 6th March 2004.
• 2004 is a leap year (divisible by 4 and by 400, since it is divisible by 4 and 100 but also by 400).
• 2005 is a non-leap year.
• We use the standard Gregorian calendar.

Concept / Approach:
The key concept is how the same calendar date shifts from year to year. Moving from a date in year Y to the same date in year Y+1 usually advances the weekday by 1 day if the year Y+1 is not a leap year and by 2 days if year Y+1 is a leap year and the date is after 29th February. When moving backward (from Y+1 to Y), we reverse this shift. Here, we go from 6th March 2005 back to 6th March 2004.

Step-by-Step Solution:
Step 1: We know 6th March 2005 is a Monday (given). Step 2: Consider how the weekday changes from 6th March 2004 to 6th March 2005. Step 3: The later year in this pair is 2005, which is not a leap year. Step 4: When the later year (2005) is non-leap, the weekday for the same calendar date advances by exactly 1 day from 2004 to 2005. Step 5: That means: weekday(6th March 2005) = weekday(6th March 2004) + 1 day (modulo 7). Step 6: We are given weekday(6th March 2005) = Monday. Step 7: Therefore, weekday(6th March 2004) must be one day before Monday, which is Sunday.

Verification / Alternative check:
Think of it in reverse: If 6th March 2004 were Sunday, then going forward one non-leap year to 6th March 2005 would shift the day by +1 to Monday. This matches the given condition exactly, confirming that Sunday is consistent with the assumed information.

Why Other Options Are Wrong:
Tuesday or Wednesday would imply a shift of more than one day when moving forward to Monday in 2005, which contradicts the rule for a non-leap year. Friday would require a large backward jump from Monday, incompatible with the one-day shift. Monday would mean no shift at all, which again is not possible between 2004 and 2005 when considering 365 days between the same dates.

Common Pitfalls:
A common mistake is to confuse whether the leap year is the earlier or the later year when determining the shift. Another error is to assume a two-day difference automatically whenever a leap year is nearby, without checking which year is leap and whether we are moving forwards or backwards. Carefully track the direction (forward or backward) and whether the later year is leap or not.

Final Answer:
The day of the week on 6th March 2004 was Sunday.