On this day the supreme Lord Sri nArAyaNa took the incarnation of Sri Hayagreeva dEvaru and restored the vEdas which was stolen by a demon. In this incarnation, His head is of a pure white horse, while the rest the body is like a human, possessing four hands. He holds shankha, japamaala and pustaka in three hands; and the fourth hand shows the jnAna mudra. Lord hayagreeva then preached the vEdas to Sri brahma dEvaru on this auspicious day for the benefit of entire mankind.
Sri Hayagreeva devaru is worshipped as the God of knowledge. He is the upAsya mUrthy of Sri vAdirAjaru. So, the day of his avatAr is chosen as the day to start and submit the vEda adhyayana; and wearing a new sacred thread is a part of this ritual.