From Jan 1, 2006 to Jan 1, 2010 is 4 years. Normal years advance the day by 1, while leap years advance by 2. 2008 was a leap year. Total days advanced: 2006→2007 (1), 2007→2008 (1), 2008→2009 (2, leap year), 2009→2010 (1). Sum = 5 days. Sunday + 5 days = Friday.