Direct answer: Koo and the Two Fools runs from September 23 at 07:00 UTC until the October 8 maintenance. Complete the Previous Story and daily story quests after clearing the Main Quest Calm Days to earn Pirate's Gold and Sea Wanderer armor.
How do I unlock the event?
Complete the Main Quest Calm Days. The Previous Story starts from the event tab at the rocky islet in the middle of the sea; after clearing Onward, the Journey Continues, the daily story becomes available.
What are the quest rewards?
- Previous Story quests: up to Pirate's Gold x180.
- Daily Story quests: up to Pirate's Gold x880, Sea Wanderer Top x1, and Sea Wanderer Bottoms x1.
- Daily quests unlock sequentially from Day 1 through Day 9 at 07:00 UTC.
What is in the exchange shop?
Pirate's Gold can be exchanged for Sea Wanderer Belt, Sea Wanderer Combat Boots, Transcendence Engraving Stones, Transcendence Refinement Stones, an SSR Crafted Weapon Selection Chest, crafting blueprints, Element Spell Bead Selection Chests, cube keys, enhancement and refinement materials, mastery EXP, and Gold.
Pirate's Gold is usable only in the event shop during the event period. Rewards cannot be claimed after the event ends, and remaining Empty Mugs are reset when the event ends.
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Official source: Event Story - Koo and the Two Fools.