Starlight Glimmer was having a pretty good morning until she discovered something that shook her to her very core: Apparently she's dating both of her best friends, and she's only finding this out on their first anniversary together.
(Prequel to "Birthday Built For Two".) On a trip to the Crystal Empire, Starlight finds Sunburst acting rather strange. Before she has a chance to ask him though, she gets roped into helping him foalsit Flurry Heart for a day.
The second changeling attack on Equestria has helped the Royal Crystaller Sunburst in getting a fresh perspective on several aspects of his life. One particular aspect comes into focus when Shining Armor and Cadance offer him a weekend off.
After the crystalling, the banquets, and the private after-party, Starlight gets her chance to be alone with Sunburst. They realize that they share two things in common: their friendship and different perspectives of the day he got his cutie mark.