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

On 31st December, 2005 it was Saturday.

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

therfore, on 31st December 2009, it was Thursday.

Thus, on 1st Jan, 2010 it is Friday