Parvati and Shiva lived on Mount Kailash, high in the Himalayas. Sometimes, though, Parvati visited the earth below, staying with Bhudevi, the Earth-Goddess.
One time when Parvati was visiting Bhudevi, Shiva grew especially lonely, so he sent Ganesha to go bring her back. "Tell your mother I miss her," he said to Ganesha.
The goddesses gladly welcomed Ganesha, and he spent eleven happy days with them. Then he remembered his mission. "Please come home," he said. "Shiva is lonely."
Parvati agreed and returned to Mount Kailash.
Ganesha's visit to earth is remembered with eleven days of celebration during Ganesha Chaturthi.
Inspired by: Stories of Lord Ganesha
Notes: You can read about the holiday of Ganesha Chaturthi at Wikipedia. Here is a photo of the celebrations by Thejas Panarkandy at Flickr: