lf 09/12/2001 happens to be Sunday, then 09/12/1971 would have been at

09/12/2001_________sunday 

No. of days between 9/ 12/ 71 & 9/12/2001 

we know every year has 1 odd days 

we know leap year has 2 odd days

 Here, No. of normal years = 22 

And no. of leap years = 8

 So odd days= 22 + 16 = 38 i.e. 3odd days (remainder when 38 is divided by 7, i.e. 3) 

Hence it was a Thursday