Question

If the 1st January of a certain year, which was not a leap year, was a Thursday, then what day of the week was the 31st December of that year ?

Answer 1

The given year is a non-leap year and contains $365$ days.

(The number of days from $1st$ January to $31st$ December is $365$)

$365\equiv 1$ $\left(mod7\right)$

The $365th$ day is equivalent to the first day of the year.

But the first day of the week is Thursday.

Hence, $31st$ December is Thursday.