It can be hard for a sibling to perceive matters from the other’s perspective. Getting Celestia to view any part of the world through Luna’s eyes takes special measures. And a lot of water.
As facts go, Celestia can always remember 'Why don't we let first-year Gifted School students have a project fair?' is a horrible idea. Everypony else usually needs about eighty years to forget that.
After Celestia's School For Gifted Unicorns runs out of repair money and no more can be found in Equestria's budget, Luna invents a means to raise the funds. Now if she can just keep it from destroying the nation's economy...
The second anniversary of the Return is approaching, and all Luna wants for the celebration is one thing -- something Equestria hasn't seen in more than a thousand years. This could be a problem.
There are things a Princess cannot do. Over the centuries, Celestia has been asked to do those things countless times. And the moments when her ponies need her the most are the ones where she can't do anything.
Following a certain eclipse, both halves of the Diarchy have agreed to work on their absent culinary skills. Anise Verum and Blending Stock are the lucky ponies who get to change that status. ('Lucky' may not be the right word.)
For nearly every citizen of Equestria, it's a holiday: a chance to get together with friends, gather a few treats, and laugh in defiance of the darkness. For Celestia, Nightmare Night is something else entirely.
The good news: having her false ID summoned for jury duty means Chrysalis' disguise is perfect. The bad: running out on it would break her cover. (Why do queens always wind up with the hard decisions?)
Keeping a newborn's uncontrolled magic from wrecking the house during a foalsitting evening requires an experienced professional! -- or a teenager who hasn't figured out what the job actually requires. (The teenager is initially cheaper.)