This page explains how to start and complete the By Firefly's Light side quest.
How to Start the Quest
Before you can start this quest, you have to complete three quests in Kakariko Village. First, you need to complete the Seek Out Impa quest and the Locked Mementos quest. Then, talk to a guard in front of Impa's house if you haven't already. Then during the day, talk to Cado, who can be found in front of the house across from the armor shop in Kakariko Village. He asks you to find his Cuccos. This starts the Flown the Coop quest. You need to complete that quest as well. You also need to talk to Pikango, who can be found across from Impa's house, and complete his Find the Fairy Fountain quest. Then at 10:00PM or later, go into the southeastern house and talk to Lasli to start this quest.
Collect at Least Five Fireflies
If you don't already have five Sunset Fireflies, you can find them at night in Kakariko Village.
Release Five Fireflies near Lasli
Go back to Lasli in the southeast house of Kakariko Village, then go into your inventory and hold five fireflies. Then exit the inventory, and press A to release the fireflies near Lasli.
Quest Reward
Lasli gives you a purple rupee for completing the quest.