Party

A Party is decoration with a small tail of rewards attached. It ends at dusk, and one that starts on its own begins at dawn, so a natural party fills the whole daylight stretch. While it runs, nearly every town NPC puts on a Party Hat and throws confetti at anyone standing near them, balloons rise through the background, and wild bunnies turn up wearing hats of their own. Gem Bunnies are replaced by ordinary ones for the duration, so if you were farming those, come back later.

The ones who sit it out are the Old Man, the Skeleton Merchant, the Tax Collector and the Zoologist. Everyone else joins in, town pets and the Traveling Merchant included, and most of them have party-only dialogue. Location does not matter either: an NPC living in a jungle outpost is wearing a hat too.

Random parties are thrown by the NPCs, rolled at dawn with a 1 in 10 chance, better in a Celebration Mk 10 world. The conditions are easy to miss. The Party Girl has to be living in the world, there have to be at least five NPCs eligible to throw one, and the last random party has to be more than 5 to 10 days behind you, with the exact cooldown picked at random each time. One to three NPCs get named in the announcement, and three names only becomes possible once you have 13 or more eligible townsfolk, which is a decent rough gauge of how built out your town is.

A Party Center skips all of it. Flip the switch and a party starts, even in a world where every NPC is dead. That is the reliable way to open the party shops, but it costs you the rewards below.

If the party started by itself, the Party Girl hands over a Slice of Cake the first time you talk to her, once per party rather than once per player, so in multiplayer somebody is going without. A natural party is also what brings the Cool Slime town pet in. Neither happens for a party you switched on yourself, so if you are collecting pets, let the game roll its own.

The Clothier and the Party Girl both open extra stock for the duration and close it the moment the party ends. It is all novelty and vanity: the Confetti Cannon, balloon gear like the Silly Balloon Machine, party clothes for the vanity slots. None of it changes a fight. That is the case for keeping a Party Center in the base, since it turns a shop you have to wait for into one you can open whenever a build needs it.

• The background balloons only render when an NPC or a Party Center is within a couple of screens of you. Walk far enough from town and the party looks like it stopped.
• A party never ends early, not even if every NPC in the world dies. It runs to dusk regardless.
• The Tax Collector wears no hat and says nothing about the occasion, but he can still be one of the NPCs who throws it.
• Celebration Mk 10 worlds open with a party already in progress. Combine that seed with For the Worthy, as a Zenith world does, and the end of that first party kills almost every town NPC in the world.
• This is the only time some NPCs appear without their usual headgear, which is why the Dye Trader looks like a different person.

The event was added for the game's fifth birthday in May 2016, which explains why the whole thing is cake and confetti rather than loot.