Avani Avittam (Tamil: ஆவணி அவிட்டம்) refers to the day corresponding to the Śravishthā nakshatra ("Avittam" in Tamil) of the Śrāvana month ("Avani" in Tamil) of the Hindu calendar when Brahmins ritually change their Yajñopavītam ("Poonool" in Tamil/Malayalam) every year accompanied by relevant Srauta "upakarma" rituals.
The day, also called Śrāvana Pūrnima (full moon day of the Śrāvana month) in other parts of India, usually occurs the day after the Śravana nakshatra (which marks the Onam festival of Kerala). The next day, usually coinciding with the raksha bandhan festival in northern and central India, the Gayatri Mantra is recited 1008 times.
In 2010, Avani Avittam occured on August 23rd/24th (depending on the span of the tithi & nakshatra). Brahmins belonging to the Sama Veda however do not perform the upakarma rituals or change their Yajñopavīta on Śrāvana Śravishthā (i.e Avani Avittam) but rather on Bhādra chaturthi (the fourth day of the month of Bhādra/Bhādrapada, which is also known as Prosṭha/Prosṭhapada)
What is done
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 (the sacred thread).
Then a new Yagnopavita is worn. 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."
Procedure of Avani Avittam
First the Rishi Tharpanam (offering prayers to the ancient Rishis) is read out.
Brahmin bachelors perform 'Samitha Daanam' and 'kamo karshith japam' after Mahasankalpam.
Thereafter, Kaanda Rishi tharpanam is performed with the help of family priest or elders.
As per the rituals, all the male members should have a light meal at night.
Next day after an early bath, 'Gayathri Japam' is read out by everyone. Thereafter, Appam & Idli are served.
Green Gram, Dhal or Kondai Kadalai Sundal are offered as Neivedhyam for the Homam (Fire ritual).
The Homam is performed either in the house or in temples and the Arti is performed after everyone returns home.
Naivedyam - Payasam, Vada, Gingely seeds, Rice. The other items that are served on this day are Curd Pachadi, Kosumalli Curry, Koottu, Pitlay, buttermilk, soup, rasam, dhal and chips, appalams.
This day is also auspicious as the Brahmins offer libations of water to their ancestors to whom they owe their birth and to the great Rishis to whom they are highly indebted for spiritual knowledge and the Vedas themselves.
Source : The hindu blog