UPSC Civil Services 2019 Prelims General Studies Paper II

Number of odd days in a non-Leap year = 1.
Numbe of odd days in a Leap year = 2.
Two year have same calender when number of odd days between two years is 7.
Between 2009 and 2015:
Number of non-leap year = 5 = 5 odd days
Number of Leap year = 1 = 2 odd days
Hence, 2015 have same calendar as 2009.