How the custom started Legend has it that there was a bull in the locality, that will pillage through the groundnut farms, spoiling the crops every full moon night.
The farmers then offered prayers to Basava (Nandi) to stop this and pledged to offer their first crop. Later an idol of a Nandi (Bull) was found nearby. Myth has it that this idol was growing rapidly and the farmer's fixed an iron ped on its forehead. The mark on the Bull image you can see now is believed to be associated with this legend.
The then local chieftain Kempegowda (considered as the founder of the city ) built a temple dedicated to Nandi, at the hillock nearby. The bull stopped ransacking the fields!
The grateful farmers offered a portion of their first harvest to the temple. The belief is that on the second day of the fair, Lord Basavanna (the divine bull) will appear in animal form and eat the offered groundnut. The custom continues even today, close to 5 centuries after the Bull Temple was constructed.
Check the news on the exact date of the fair. It's held typically around November-December every year.
The fair is an opportunity to see the traditional village fair with tingrets stall, street food stalls and all the paraphernalia associated with a traditional harvest fair.
The date for the fair is the last Monday of Karthika month (month in Hindu calendar).