Pinkie Pie had made the promise; to let Diamond see the coming Midsummer festival, where Princess Celestia is raising her Sun. Pinkie Pie does not break promises.

The festival happens to be in the Crystal Empire this year; but that doesn’t stop Pinkie Pie from living up to the one promise. If you promise something; you Pony up and deliver on that promise, no matter what.

Naturally; Diamond had been looking forwards to the event, the excitement beaming through her eyes. Now; there is the one detail about the custom she was told to wear while outdoors, which is extending even to the Empire; but what is that compared with the chance to actually see the Sun Raise?

An adventure has to have a highlight, but they never told about the surprise that came right along with it.