If you have 30 Fireflies in your Inventory, then the first part of this quest will already be completed. If not, then send some Fireflies to your neighbours as Gifts, and they should return the favour.
When you click on the Carnival Pavilion to complete the second part of the quest, the 30 Fireflies will be removed from your inventory. It will then take 16 hours to hammer these into place on the construction. You can Speed Up the hammering with 2,088 Food.
"We're almost ready for the party! I hope the King enjoys it."
As you complete the quest, you will receive another letter from the Gnome King.