By Firefly's Light
You have to have completed the following two Kakariko Village sidequests to unlock this sidequest:
You can click on those two sidequests to see their respective pages and learn how to complete them.
After 10:00 PM, speak with Lasli inside the house nearest Kakariko Village's entrance. She longs to see fireflies.
Go into your inventory and let 5 Sunset Fireflies loose in her house. If you don't have any Sunset Fireflies, they can be gathered anywhere with lush, green grass and forests (basically anywhere that is not too hot or cold) at night. Be sure to sneak up on them.
Lasli will admire the Sunset Fireflies and give you a purple Rupee worth 50 Rupees to show her gratitude. If you want to grab those Sunset Fireflies back after showing them to Lasli, you can.
This sidequest is a prerequisite for the Shrine Quest "The Stolen Heirloom".