Calendar arithmetic — If 1 January 2006 was a Sunday, what day of the week was 1 January 2010?

Introduction / Context:
We move across four New Year boundaries to find the weekday on 1 Jan 2010 given 1 Jan 2006 was Sunday.

Given Data / Assumptions:

• Years traversed: 2006→2007 (non-leap), 2007→2008 (non-leap), 2008→2009 (leap), 2009→2010 (non-leap).
• Shifts: non-leap +1, leap +2.

Concept / Approach:
Add the annual shifts modulo 7.

Step-by-Step Solution:
Total shift = +1 (2006) +1 (2007) +2 (2008 leap) +1 (2009) = +5.Sunday + 5 → Friday (Sun→Mon 1, Tue 2, Wed 3, Thu 4, Fri 5).

Verification / Alternative check:
From 2006-01-01 to 2010-01-01: 4 years with one leap year → 1461 days; 1461 mod 7 = 5.

Why Other Options Are Wrong:
Tuesday/Wednesday/Thursday correspond to +2/+3/+4 shifts, not +5. Thus none matches Friday.

Common Pitfalls:
Forgetting 2008 is leap; miscounting the number of +1 steps.

Final Answer:
Friday, therefore “None of these.”