The Supreme Lord appeared as the son of Kashyapa and Aditi in the form of a dwarf Brahmana (Vamana) to deliver Bali Maharaja, a great devotee born in the family of demons.
Sri Jayadeva Gosvami sings:
chalayasi vikramane balim adbhuta-vamana
kesava dhrta-vamana-rupa jaya jagadisa hare
O Kesava! O Lord of the universe! O Lord Hari, who have assumed the form of a dwarf-brahmana! All glories to You! O wonderful dwarf, by Your massive steps You deceive King Bali, and by the Ganges water that has emanated from the nails of Your lotus feet; You deliver all living beings within this world.
At ISKCON, Mysore on Vamana Jayanthi the Utsava Deity of Lord Sri Krishna is decorated as Vamanadeva wearing a deer skin and holding a staff, an umbrella and a water pot (kamandalu). As the Lord goes to seek alms from Bali Maharaj on this day, devotees bring a lot of alms in the form of rice, dhal, jaggery, dry fruits etc… and offer it to Lord Vamanadev on this auspicious day.