13th June

On this day in 1771, Lagrange presented the first proof of Wilson's theorem to the Berlin Academy:-

A number p is prime if and only if $1 + (p - 1)!$ is divisible by p.

Edward Waring published the theorem in 1770, but Leibniz knew it previously.

The postage stamp of one of today's mathematicians at THIS LINK was issued in 1991.