Gaur poornima is the appearance day of lord sri chaitanya mahaprabhu, the hidden avatar of lord krishna. Shri chaitanya Mahaprabhu appeared to inaugrate the sankirtan movement for saving the fallen conditioned souls of kaliyuga. Sri Chaitanya Mahaprabhu predicted that one day the holy name of lord hari will be chanted in every town and village. His divine grace A.C. Bhaktivedanta Swami prabhupada fulfilled this prophecy by going to west in the advanced age and preaching the message of krishna conciousness vigouresly. This day is celeberated in a grand way in every ISKCON center because ISKCON is founded on the principles and precepts of shri chaitanya Mahaprabhu.