30 years gap with 8 leap years( 72,76,80,84,88,92,96,2000)
in every year 1 day is forwarded 365 mod 7 =1
in every leap year 1 extra day is forwarded 366 mod 7 =1
so total days difference 30+8 =38 days
38 mod 7 =3 so 3 days are forwarded from 9/12/71 to 9/12/01
hence it was Thursday ( 3 days backward) on 9/12/71 since it was Sunday 9/12/01