What is the name of the princess who rules the night?

The princess who rules the night is Princess Luna.

Luna is an alicorn who governs the night alongside her older sister, Princess Celestia, who raises and oversees the sun. Luna’s royal responsibility includes raising the moon and protecting the nighttime realm, including helping ponies through their dreams.

Her history explains why the question can cause confusion. Luna became resentful when ponies slept through the night and failed to appreciate her work. That bitterness transformed her into Nightmare Moon, who attempted to create eternal night. Celestia used the Elements of Harmony to banish her to the moon.

After Twilight Sparkle and her friends defeat Nightmare Moon in the two-part premiere, Luna is restored and reconciles with Celestia. Princess Cadance is associated with love and the Crystal Empire, while Twilight later becomes the Princess of Friendship; neither is the night’s royal guardian. Princess Luna is the canonical answer.