On January 12, 1980, it was Saturday. The day of the week on January 12, 1979 was -

The year I 979 being an ordinary year, it has 1 odd day. 

So, the day on 12th January I 980 is one day beyond on the day on I 2th January, 1979.

 But, January 12, 1980 being Saturday.

$\therefore$ January 12, 1979 was Friday.