Srimati Radharani appeared on the eighth day of the waxing moon of Hrsikesha (Bhadra) mase, at noon.
In Goloka Radharani goes to Barshana (Varshan) for Her Appearance day celebrations.
2007 -> Radha ashtami falls on Sep 20, 2007
As per Iskcon Vrindavan Calendar:
20 Sep 2007 Th Radhastami: Appearance of Srimati Radharani
(Fasting till noon)