Shravan Purnima is an auspicious day for the Brahmins because on this day Lord Hayagriva, the protector of the Vedas, was born. Lord Hayagreeva also referred as the God of wisdom and knowledge. During the observance of Aavani Avittam or Upakarma, Sacred Thread ceremony (Yagnopaveetha Dhaarana or Janeyu Dhaarana) is performed. This ceremony is observed as the Vedic rituals. On this day, children start their Vedic studies. The day after Avani Avittam is observed as Gayathri Japa Day.

 Avani Avittam

About Avani Avittam Rituals
On this day, a Mahasankalpam or a vow is taken for atonement of all our sins in the past year. The Brahmins take a holy dip and wear a new holy thread called Yajnopavit or Janeyu.

The first step is a 'prayashchita', A prayer to atone the sins. It says, For the removal of all my sins and thereby to secure a divine blessing and for qualifing myself to perform the essential duties of Brahmanas as prescribed in the vedas and smritis and adopted by the really good in their conduct I put on this Yagnopavita. 

When the thread is worn another mantra is recited which means -I put on the sacred thread which is highly pure, is inseparable from God, is capable of prolonging life and is the foremost in the accomplishment of a Brahmana. May such pure Yagnopavita bring strength and dignity.

While removing the old thread, the mantra means -I throw away the broken dirty old thread, may the new one bring on long life and Brahmana's brilliance. 

Avani Avittam Significance
Upakramam means the beginning. On this day the Yajurvedis begin to read Yajur Veda for next six months. The day is auspicious because as per Indian mythology Lord Vishnu was incarnated as Lord Hayagriva,the lord of knowledge, the one who restored the Vedas to Brahma.