An idol of Goddess Gowri (Goddess Parvati) is worshipped on the Gowri Habba day. Usually the idol is prepared using Turmeric or a clay idol of Goddess Gowri is also procured from the market. There are several regional variations in the observance. In some places, a clay idol of Ganesha is also worshiped. The main legend is associated with a creation of Ganesha from her own body Goddess Gowri.

More info from wiki

This festival celebrates Goddess Gowri (also known as Parvati), venerated as the mother of Lord Ganesha. It is usually observed by married women and is a significant festival in parts of Karnataka and Tamil Nadu. It is known as Hartalika in the North Indian states of Uttar Pradesh, Bihar, Jharkhand, Rajasthan, Chhattisgarh, Maharashtra and Madhya Pradesh. Ma Goddess Gowri (Gauri), the mother of Ganesha and wife of Shiva, is worshiped throughout India for her ability to bestow upon her devotees courage and power. Hindu belief has it that Goddess Gowri is the very incarnation of Aadhi Shakthi Mahamaya. She is the Shakthi of Lord Shiva. It is believed that on Thadige, or the third day of the month of Bhaadra, Goddess Gowri comes home like any married woman comes to her parents’ house. The next day Lord Ganesha, her son, comes as if to take her back to Kailasa.