Kedareshwara Vrata as the name indicates is dedicated to Lord Shiva. It is observed on the Amavasi or No moon day occurring during Diwali. Kedareshwara Vrata 2012 date is November 13. Earlier, Kedareshwar Puja used to begin on the Ashtami day of the Krishna Paksha of Ashwin month and end on Amavasi day. Now most people only observe the Kedareshwara Vrata on the Ashwin Amavasi day.
Legend has it that Kedareshwara Vrata was observed by Goddess Parvati to merge with Lord Shiva and be part of his body.
Once a Gana (attendant of Lord Shiva) performed puja of Lord Shiva and did Pradakshina or circumambulation of Lord Shiva. The Gana did not do Pradakshina of Goddess Parvati.
Goddess Parvati asked Shiva why the Gana ignored Her. Shiva was in a playful mood and said that those who know the Supreme Being have nothing to do with Parvati and so he did not do the circumambulation of Her.
Goddess Parvati got angry and left Kailash and started living in a forest on earth. She met Gautama Rishi at the forest and wanted to know how to be part of the body of Lord Shiva. Sage Gautama advised her to perform Kedareshwara Vrata.
People observe a fast on Kedareshwara Vrata day. Some people only do a partial fast. This Vrata is observed for attaining Moksha and merging with the Supreme Being.