Vagh Baras also known as Wagh Baras is observed on the 12th day during the waning phase of moon ( ie the krishna paksha) in the month of ashwin as per the traditional calendar followed in Gujarat and Maharashtra. The corresponding month in North Indian calendar being the Karthik month. In the year 2012, this is celebrated on November 10th 2012.
In Gujarati, Vagh means repaying ones financial debts. The significance of this festival is that on this day people clear their ledgers known as Khata and enter into new transactions only after Labh Panchami. They want to bring back all the wealth into the house during this period and hence this tradition of clearing debts has arised. However for practical purpose, nowadays, only the ledger entries are renewed and the actual financial transactions continue normally.
On this day as part of the ritual they worship the cow and calf and this vrat is known as the Nandini Vrat in Dhodia tribal villages in Gujarat in great fashion. Also it is widely believed that Shri Vallabha who was an incarnation of Lord Dattatreya left the earth on this day. Hence it is also celebrated as Guru Dwadashi in some places.