The Star Festival is a special event that occurs once every century in the Mushroom Kingdom. The festival is held by the kingdom's citizens in celebration of a special comet that passes through the skies above the Mushroom Kingdom once every one hundred years. One of these centennial Star Festivals was featured in the game Super Mario Galaxy, and the event would serve as the basis for Mario's adventure across the universe.
During the events of Super Mario Galaxy, the Toadsof the Mushroom Kingdom gathered in the Castle Gardens outside Princess Peach's castle to celebrate the centennial Star Festival. This year, the comet released a variety of shooting stars, in the form of star bits, that landed on the ground below. During the festival, the Toads enjoyed collecting these Star Bits that had fallen from the sky. Some of these shooting stars even combined to form a powerful grand star which was brought to Peach's Castle by the Toads. Princess Peach even stumbled upon a special star creature named luma.
Mario was invited by letter to this particular Star Festival by Princess Peach herself, who wanted to present her friend with a special gift, Luma. Mario arrived just in time for the festivities, and indeed was the last guest to arrive at the Castle Gardens for the centennial celebration. All was going well, until Bowser crashed the party. With a fleet of airships, Bowser launched an assault on the Mushroom Kingdom, eventually abducting Peach's Castle with the Princess inside. With a special flying saucer, Bowser transported his fleet and the castle to the center of the universe. This assault was part of Bowser's plan to conquer the entire universe, and rule a great galactic empire with Peach by his side.
Bowser was using the power of the Grand Stars to re-create the universe as he saw fit. These Grand Stars were stolen from the Comet observatory, a mobile space station inhabited by , the protector of the cosmos. In fact the "comet" celebrated every century by the Mushroom Kingdom was actually this mobile space station. Rosalina would fly the Comet Observatory by her home planet every one hundred years, and this particular year Bowser attacked the space station and stole the Grand Stars that powered the Observatory. Indeed, the raining of shooting stars this year may have been caused by Bowser's assault on the Comet Observatory.
After the universe is saved from Bowser and reconstituted by the lumas, and both Mario and luigi have each collected all 120 power stars, the two can travel to the Grand finale galaxy, the location of their home world and the Mushroom Kingdom, to celebrate a second attempt at a Star Festival. Like before, Mario and Luigi arrive just in time for the festivities. Many of the characters Mario and Luigi encountered in their journey across the cosmos appeared in the Castle Gardens, celebrating the centennial Star Festival with the Toads