Hey, it's the end of the day! She's wearing her night-dress!
BTW - Every night it is a randomly-chosen Day Guard's duty to see to it that the Princess is... serviced at the end of the day, an ancient tradition dating back to shortly after the fall of Discord when just about everypony thought that Celestia and Luna were literally divine and the then-teenage Princesses were hormonal enough to really, really like the idea of a stallion a night. It's little more than a ritual now, with the guard required to kiss the Princess and then remain in the room with her all night and attend to any wishes she may have. A guard may find themselves listening to the latest Friendship Report from one of the Elements of Harmony. He or she might be asked their opinion of the day's events or on some pressing matter of state. He or she might even find themselves challenged to a game of chess, checkers or (and Celestia is mercilessly good at this game) Hungry Hippos.
However... for some lucky(?) guards whose beauty or nobility of spirit particularly catches Celestia's attention... the reward for their loyalty to duty is far greater. There is a reason why Celestia was able to tell Cadence that she was a 'very, very lucky filly' at the wedding reception.
"'Dear Princess Celestia... Blah-blah... Expression of neurotic insecurity... Blah-blah... Obsessive-compulsive failed attempt to fix problem caused by my own panicked stupidity... Blah-blah... Friendship saves the day... Blah-blah... Lots of love, Twilight.' Thank the Sun I got that girl out of the castle! With luck, one of her friends will get her to loosen up before she breaks something!"