The Star Festival
From the Super Mario Wiki
The Star Festival is the only mission of the Grand Finale Galaxy in Super Mario Galaxy, named after the Star Festival from the same game. This mission can only be done if both Mario and Luigi have obtained 120 Power Stars. The mission's objective is to collect 100 Purple Coins, and this Purple Coin mission does not have a time limit.
One hundred Purple Coins are scattered around Toad Town and the castle garden. The mission has no obstacles or enemies (though the player can still die by standing in fire or drowning), so the player just needs to simply collect all of the coins. Once they have all been collected, the Power Star will appear in front of the castle.
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