Rasha Ashtami is commemorated as birth anniversary of Goddess Radha, the consort of Lord Krishna. It falls on Ashtami Tithi during Shukla Paksha of Bhadrapada month.
Devotees keep fast on Radha Asthami day. Goddess Radha is worshipped during Madhyahna Kala which is noon time according to Hindu division of the day. Currently Radha Ashtami falls in the month of August or September in English calendar.
Radha Ashtami is also known as Radhashtami and Radha Jayanti.