By Firefly's Light
Lasli is available inside her house (the closest to the entrance to town) at night, after approximately 10 PM.
The goal here is to let 5 Sunset Fireflies loose inside her house, so first you'll have to catch some if you don't already have them. They will fly around Kakariko at night, although typically closer to the midnight through 3 AM timeframe unless it's raining. Teleporting away and back can respawn them. They will disappear if they detect you, so you have to sneak up on them. They're also available elsewhere in Hyrule.
Once you have five, release them in Lasli's house while she's there to get your reward, a purple rupee.
Tip: If you have done the Slated for Upgrades Side Quest and have Sensor+, take a picture of one of a Sunset Firefly. You can then set it as your Sensor+ target for help in finding more of them.