Rarity happens upon a centuries old painting of Princess Luna in the Sisters' old castle in which she was wearing a "gorgeous" gown. Upon hearing Celestia mention how the gown was one of her little sister's favorites, Rarity extends an offer to recreate it for her.

This scene is after Rarity asked Luna to remove her regalia so she could get her measurements. Naturally she feels uncomfortable due to not usually being this exposed.
——————

Background: https://boneswolbach.deviantart.com/art/Canterlot-Room-553739176

Rarity: >>251877

Luna: >>1669918