Neptune was "officially" discovered on August 10, 1846; Friday marks its first completion of an orbit since then. Actually, Gallileo Gallilei saw Neptune in 1612 and 1613, but mistook it for a star because it's very small in the tiny scope Gallileo had.

See here.

I've observed it several times in my 11". Not much to see, as even in that scope (280mm vs. Gallileo's 40mm), it's still quite tiny, barely resolved as a disk.