Kedareshwar Vrata, also known as Kedara Gauri Vrat, is observed on the Amavasi day during Diwali. Kedareshwar Vrata 2012 date is November 13. Kedara Gauri Vrat is observed on Kartik Amavasi in
North India and the corresponding Amavasya in other regions is Ashwin Amavas. The Vrat is dedicated to Lord Shiva and is based on Goddess Parvati's attempt to merge with Lord Shiva.
Initially Kedareshwar Vrata used to begin on the Ashtami day or eighth day of the Krishna Paksha of Ashwin or Kartik month and end on Amavasi day during Diwali. But now most people only observe the Vrat on the Amavas day.
Most Shiva devotees observe a fast on Kedareshwara Vrata day. Some people only perform a partial fast.
Kedareshwar Vrata is observed for attaining Moksha and merging with the Supreme Being.
You can read about the legend associated with Kedareshwar Vrata here in an earlier article on the same topic.