Vamana Jayanthi is the first appearance day of Lord Vamana, the 5th and the dwarf avatar of Lord Vishnu. Vamana Jayanthi 2011 date is September 8. This is the first complete human incarnation of Sri Maha Vishnu. It is observed on Shukla paksha Dwadashi (12th day in bright half) of Bhadrapad month. This festival is different from Vamana Dwadashi Puja which is observed on Ashad Shukla Dwadashi day.
The main purpose of Vamanavatara was to restrain the demon king Maha Bali and to restore the rule of Gods on the earth. That is the main reason why Vamana sent Bali to Pathala loka. You can read the legend or story of Vamana avatara here.