Today , $5^{th} April, 2018 \rightarrow Thursday$
$\qquad 5^{th} April, 2017 \rightarrow Wednesday$
$\qquad 5^{th} April, 2016 \rightarrow Tuesday$
$\qquad 5^{th} April, 2015 \rightarrow Sunday$
$\qquad 5^{th} April, 2014 \rightarrow Saturday$
Clearly, we can see that when we move down yearwise, a day decreases, but when a leap year came, $2$ day decreases.
Now, the year $1979$ being an ordinary year, it has $365$ days, means $1$ ODD day.
Now, $12^{th}Jan, 1979$ is $Saturday$.
From $12^{th}Jan, 1979$ to $12^{th}Jan, 1980$ is a total of $365$ days.
∴ $12^{th}Jan, 1979$ will be $1$ day before $Saturday$ & it will be $\color{Orange}{Friday}$