A Midnight Flight

A Midnight Fly is an anomalous level not present in most copies of Super Mario 64, and has received little documentation. Across all documentation, it has appeared as a victory lap of sorts for when the player has completed the game or collected all 120 stars. The level looks exactly like Princess Peach's Castle, only that the level takes place under a night sky and many of the textures have been tinted blue. This level is primarily built around the Wing Cap.

Details on the level have been aggregated from many different sources.

Mission
This level only contains one mission, which is the same as the name of the level. "A Midnight Fly". Oddly, there is no star at the end of this level, and instead the end contains a sign that reads:

"Thank you for your time with Super Mario 64. At Nintendo we strive to make memorable games for our players, and we hope you had fun.

Sometimes it's good to take a break, though!

-The Mario 64 Team"

This message varies across copies, but generally says this.

The level contains a series of rings that differ in color across copies, but are usually vibrant colors. Mario must pass through these rings with the Wing Cap in order to find a sign on a cloud on top of the castle.

Level Geography / Accessing the Level
In order to access A Midnight Fly, the AI must think you are ready for the level. If it thinks you are sufficiently ready, a yellow toad will appear in front of a new door that leads outside the castle. The toad says:

"Mario, would you like to take a midnight fly around the castle? Head into the door!"

The door probably appears due to Dynamic Level Rearrangement.

The level starts on the platform where you would usually find Yoshi. There is a Wing Cap Box right outside of the door. The level's geometry is almost exactly like Peach's Castle, however there is a cloud platform on top of the castle and there are many rings throughout the level (see section 2).

The AI's Choice
Every user who has documented this level describes their play-style as "slow and thorough", taking their time in levels to explore every nook and cranny for coins, secret stars, and other hidden items.

This level appears to be meant for those who like to take their time when playing Mario 64, and would be happy to take just a few minutes to take a fly around Peach's Castle with the Wing Cap.