On 8 February 2005 it was a Tuesday. What was the day of the week on 8 February 2004 in the Gregorian calendar?

Introduction / Context:
This question again uses relative dates, but now you must move across a leap year boundary. You are told that 8 February 2005 is a Tuesday and asked to find the weekday for 8 February 2004. The presence of a leap year between these dates makes this a useful test of whether you understand how leap years change the weekday shift between identical calendar dates in adjacent years.

Given Data / Assumptions:
- 8 February 2005 is Tuesday.
- We must find the weekday for 8 February 2004.
- Calendar used: Gregorian.
- 2004 is a leap year (divisible by 4 and not a century year), and 2005 is a non leap year.

Concept / Approach:
Between the same date in two consecutive years:
- A non leap year contributes a 1 day shift in the weekday.
- A leap year contributes a 2 day shift in the weekday when moving past February, because it has 366 days.
We are moving from 8 February 2004 to 8 February 2005 across the leap year 2004. The total shift depends on how many days the year contributes between corresponding dates.

Step-by-Step Solution:
Step 1: Recognise that 2004 is a leap year; therefore, the full year has 366 days.Step 2: However, when considering the shift from 8 February 2004 to 8 February 2005, we effectively cross exactly one full year.Step 3: A year of 366 days is 52 weeks plus 2 extra days, so it causes a 2 day shift in the weekday between identical dates.Step 4: If we move forward from 8 February 2004 to 8 February 2005, the weekday advances by 2 days.Step 5: We are told that 8 February 2005 is Tuesday.Step 6: To find 8 February 2004, we move 2 days backward from Tuesday.Step 7: One day before Tuesday is Monday, and one day before Monday is Sunday.Step 8: Therefore, 8 February 2004 was a Sunday.

Verification / Alternative check:
Think of it in the forward direction again: start at Sunday on 8 February 2004, add 366 days (52 weeks and 2 extra days), the weekday moves 2 days forward, landing on Tuesday for 8 February 2005. This agrees exactly with the information given in the question, confirming the backward calculation.

Why Other Options Are Wrong:
If you mistakenly treat 2004 as a non leap year, you would assume only a 1 day shift and might choose Monday instead. Alternatively, a miscount of 3 days would lead you to Saturday or Wednesday. However, the leap year status of 2004 forces a 2 day shift, and only Sunday fits that requirement when going backward from Tuesday.

Common Pitfalls:
The most frequent mistake is forgetting that February in a leap year has 29 days, not 28, and that the extra day increases the weekday shift. Some students also mix up the direction when moving in time, adding days when they should subtract or vice versa. Always decide whether you are moving forward or backward in time and apply the correct number of weekday steps accordingly.

Final Answer:
The day of the week on 8 February 2004 was Sunday.