It's almost never happened before. Scratch that, it has NEVER happened before. What just happened is rarer than a forgotten alicorn princess descending from the stars.

Pinkie Pie forgot about a party.

Down in the dumps and not feeling very much like Pinkie, the poor mare is without a clue on what to do. As luck would have it, Applejack knows just what to do to make her friend feel like herself. Good thing she's had practice.