The celebration is located close to Phil's Bar it basically lists all of the favors that you can complete. They are either party supplies which earn you party points and sand coins or minion unlocks that either welcome a particular minion to the island (welcome baskets) or unlocks a new task or story for a particular minion.
You should check these regularly to increase your coinage and party points and to unlock new elements in the game.
The party supplies and minion welcome packs look something like this. Basically you are shown what you need to collect or make and then you click on the paper with the blue tick as as it shows you have the correct items for you to complete that supply.