What was the day of the week on 31 January 2009?

Saturday

Using Code Method :-

\(\frac{Date+Month code+Year code+Last 2 digits of yr+Leap yrs till given yr}{7}\)

(Note: Remainder of the above sum will be the code of week day}

31 Jan 2009;

31 + 01 + 06 + 09 + 02 = 49

remainder of \(\frac{49}{7}\) = 0

Therefore, the code of 0 is Saturday