Avani Avittam or Upakramam is a ritual followed by the Brahmins who read Yajurveda. It is most commonly celebrated in southern states of Tamil Nadu and Kerela, and states of Orissa and Maharashtra. It falls on the Shravan Purnima that is the same day as Raksha Bandhan.
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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.

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.