About 165 years ago, astronomers figured out that Neptune is a planet. Since then, Pluto has lived and died as a planet, but Neptune is just now finishing its first solar cycle: It'll turn 1 in Neptune years on July 12.
Neptune is the only planet in our solar system that can't be seen with the naked eye (now that Pluto got the boot), so you'll need a good pair of binoculars if you want to check it out. It's the last chance you'll have to wish the the other blue planet a happy birthday for another 164.79 Earth years. [EarthSky]