Location
To start the anniversary event you need start in the lobby level and follow the arrows to the ladder room, like you would in a normal playthrough.
The starting room now looks different with partygoers and tables around.
How To Beat The Event
On the tables are darts that you can pick up and equip.
After collecting some darts, head for the table in the center and blow the candles to start. A door will appear where you can run out and start hunting the partygoers.
Partygoers have three balloons that need to be popped to be stopped.
You can pop these balloons by shooting your darts, either by running up to them, or by range. If you do attack by range, they won't run away.
If you run low on darts you can always pick them up. They appear as a shining objects that flash.
Hunting all of the partygoers may be challenging so I recommend calling out where any are to friends using the different colored lights. I also recommend picking up the moth jelly that is in the lobby.
After hunting all of the partygoers, go back to the ladder room to collect a prize. You can choose any from the four partygoers holding gift boxes to get the prize. Don't worry about repeats as once you get one cosmetic, you can't get it again!
Cosmetics
These are all four cosmetics you can get for completing the event.
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2992385354
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