If 1st Jan 2001 is Monday then which day was on 1st Jan 2013.

Number of years between 2001 to 2013 is 12 years (2013 - 2001 = 12 years)
Number of leap years between 2001 to 2013 is 3, since a year has 1 odd days and a leap year has 2 odd days , so total number of odd days is 12 + 3 = 15 odd days.
When we divide 15 by 7 remainder is 1 or in total we have 1 odd day so 1st jan 2013 is Tuesday.