Perigee

A perigee is the point in a satellite's orbit in which it is closest to the body around which it orbits. On Alternia, points at which both the Green and Pink Moons are at their closest to Alternia at once are referred to as bilunar perigees.

Twelfth perigee
The day before the twelfth perigee of a sweep on Alternia is referred to as 12th Perigee's Eve, and is celebrated as a holiday throughout the Alternian Empire.