That a Day on the Moon is the same as a Year on the Moon?
A day is defined as a celestial body's period of rotation around it's axis.
A year is defined as a celestial body's period of revolution around whatever it's orbiting.
The moon is in synchronous rotation - both of these periods are 27.321 days, which is why the moon always shows us the same face.
Which, of course, led to that dreary Pink Floyd album.
(There's currently a furious debate going on between our help desk manager and LHS as to how this can be so. I hope it doesn't come to blows... )