If 11^th August 1985 was a Sunday, then which day of the week was 13^thAugust 1986? 

If 11 August 1985 – Sunday

Date + Month code + Century code + Number of years other except given year + Number of leap years except given year + Leap year effect

$11+2+1+84+21+0=119$

$\frac{119}{7}$ remainder is ‘0’ – Sunday then 

13 August 1986

Date + Month code + Century code + Number of years other except given year + Number of leap years except given year + Leap year effect

$13+2+1+85+21+0=122$

$\frac{122}{7}$ remainder is 3 - Wednesday