It was Sunday on January 1st, 2006. What was the day of the week January 1st,…

Key idea: count how many weekday shifts (odd days) occur between the two dates.

1. There are four full years between January 1, 2006 and January 1, 2010 (2006→2007, 2007→2008, 2008→2009, 2009→2010).

2. Identify leap years among these: 2008 is a leap year; 2006, 2007, and 2009 are ordinary years.

3. Each ordinary year advances the weekday by 1 day and a leap year advances it by 2 days. Total advance = 3×1 + 1×2 = 5 days.

4. Starting weekday: Sunday (given). Add 5 days: Sunday → Friday.

Answer: January 1, 2010 was a Friday.