The Dap of the Heavens is a natural phenomenon that occurs in the Land of Ooo only every 20 years. In the episode "The Mountain," before Finn and Jake adventure into the Mountain of Matthew, they were camping out nearby getting ready to witness this phenomenon.
The Dap of the Heavens involves two large star constellations in a humanoid shape. During this event, the star at the "hand" of each constellation lights up bright enough to become a "fist" which "knocks" against the other, like a fist bump, before returning to its original position and appearance.
It can be assumed that this phenomenon can only be viewed from a highland area near the Mountain of Matthew. It requires a telescope to be able to be seen.
While it is a very rare event, only happening every 20 years, Finn commented that it was slightly disappointing, despite the fact that at first he was quite excited to see it.
- "Dap" is a slang term for "fist bump" of unknown etymological origin.