It was Sunday on Jan 1, 2006. What was the day of the week Jan 1, 2010?

On 31^st December, 2005 it was Saturday.

Number of odd days from the year 2006 to the year 2009 = (1 + 1 + 2 + 1) = 5 days.

 On 31^st December 2009, it was Thursday.

Thus, on 1^st Jan, 2010 it is Friday.