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

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

On 1st Jan 2006, it was Sunday.

Thus, On 1st Jan, 2010 it is Friday.