Calendar arithmetic — If 1 January 2001 was a Monday, then what day of the week was 1 January 2013?

Introduction / Context:
We must move 12 full years ahead, counting leap years, to find the day on 1 Jan 2013 given 1 Jan 2001 was Monday.

Given Data / Assumptions:

• Span: 2001 to 2012 inclusive = 12 years.
• Leap years in span: 2004, 2008, 2012 (3 leap years).
• Shift per non-leap year = +1 day; per leap year = +2 days.

Concept / Approach:
Total shift = 9*(+1) + 3*(+2) = 9 + 6 = +15 days; 15 mod 7 = 1.

Step-by-Step Solution:
Net weekday shift = +1.Monday + 1 = Tuesday.

Verification / Alternative check:
Compute days = 12*365 + 3 = 4383; 4383 mod 7 = 1, confirming +1 shift.

Why Other Options Are Wrong:
They imply +2/+3/+5 shifts, inconsistent with the correct +1.

Common Pitfalls:
Missing the leap year 2012 or misapplying the +2 shift rule.

Final Answer:
Tuesday.