Of the many NPCs players get to know in Animal Crossing: New Horizons, Celeste is one of the most enigmatic. The sister of the Museum owner, Blathers, Celeste enjoys gazing up at the stars and educating players about the night sky's wonders. While Celeste can be found wandering the island at night, some players may wonder where she goes when the sun comes up - and whether they might be able to find her hidden away somewhere on their islands.
Celeste appears on players' islands on any clear night between 7 PM and 4 AM. She will also arrive on the nights of Meteor Showers. If players track her down, she will give them one DIY recipe per visit for Animal Crossing Wands, Star Fragments, and Zodiac Fragments. When shown an item players have made with star fragments, she will tell them a short, mythological story.
It is possible that Celeste spends her days on another Animal Crossing island after she leaves a player's island in the morning. Other villagers can be found wandering the mystery islands that players visit, so she may spend her daylight hours going from place to place, looking for the best locations to view the night sky. She may also have a home of her own somewhere. However, it seems odd that she would go so far away, only to continue to return to the player's island so frequently.
Animal Crossing: New Horizons' Celeste Vanishes During The Day
Another theory is that Celeste could live with Blathers in the museum. In past Animal Crossing games, she has helped her brother run the Museum. According to the Fandom Animal Crossing Wiki, she has run the Museum's observatory, as well as the gift shop and exhibit rooms in other Animal Crossing titles. It could be that, when the sun rises, she returns to the museum and seeks out the hospitality of her brother's home to get some sleep, before again losing herself in the stars when night falls.
While it is possible Celeste simply comes and goes from the island like any other Animal Crossing NPC, such as Flick or Gulliver, her appearances feel almost magical. Nothing has been confirmed about what fans may expect from future updates, but it could be fun for Animal Crossing: New Horizons to add more information about the stargazing owl and potentially add her back into the daily operations of the Museum, allowing players to visit her and Blathers together during the daylight hours.
Source: Fandom Animal Crossing Wiki