Spawn Conditions

The Empress of Light is summoned by killing a Prismatic Lacewing, a rare critter that spawns in the surface Hallow at night between 7:30 PM - 12:00 AM once Plantera has been defeated in the current world. The Prismatic Lacewing is one of the few entities that can be detected by the Lifeform Analyzer or its upgrades.
The Prismatic Lacewing can be killed at any time in any biome to summon the boss. However, if released outside of the Hallow biome it will despawn near instantly, so it must be placed onto a persisting and damaging projectile or by using a weapon that could attack and kill it before the critter vanishes.
Although the Prismatic Lacewing only spawns naturally in worlds where Plantera has been defeated, it is possible to bring one to an artificial hallow in Pre-Hardmode and kill it to summon the boss.

Stratigies
• For nocturnal bosses, starting the battle just after the sun sets (7:30 PM) will provide the maximum amount of time to defeat the boss. In Hardmode, the Moon Charm and/or Moon Stone and their upgrades are also useful at night.
• Boss battles can be won or lost before the battle ever starts. Choose your equipment and have it ready; find and/or make potions and placed buffs; use the highest-tier food you have available; set up an arena appropriate to the boss; learn the strategies for the boss you're facing.
• Prepare your Boss-fighting arena with adequate lighting, Campfires and  Heart Lanterns for health regeneration, and  Stars in Bottles for mana regeneration if you use magic. Bast Statues will provide a significant defensive boost. Garden Gnomes and (if underground) proper torch placement will reduce damage taken and increase damage dealt due to the luck. Note Torch God's Favor helps with this. Honey pools can also be used for extra health regeneration.
Sunflowers provide the Happy! buff to players within a 50-tile radius, which grants a +10% movement speed bonus and 17% reduction in enemy spawn rate.