You might sometimes find Celeste on your island at night. She gives you recipes that use star fragments.
When Celeste Can Appear
Celeste can appear between 7PM and 4AM. She doesn't prevent Wisp from appearing. She can only appear once per week. She cannot appear on the same day that K.K. Slider visits, even after he leaves for the day.
Wish on Shooting Stars
If Celeste is on your island, there is a good chance of seeing shooting stars. Be sure to listen for shooting stars and make a wish!
Get Star Fragment Recipes
Talk to Celeste to get a DIY recipe that uses star fragments. If you wish on shooting stars, you will be able to find star fragments on your beach the next day, which you can use in the recipes from Celeste.