It is common among Tamil speaking people, to refer to the month by the solar month followed, As Chittirai, vaikasi etc, names evolved by observing the full moon day in the month and the star in which full moon occurs. Similarly the lunar months are alsonamed after the star in which full moon occurs. There is a slight difference in such observations:
3rd month Aani (anooraadha/anusham) -Jyeshtha (Jyeshthaa-kettai)
The Northern lunar month ends on full moon and the Southern lunar month ends on Amavasya. Thus, there is a difference of a fortnight, between the two systems. But the bright fortnight (shukla paksha is common to both and thus the month will be the same.
For followers of solar system, the lunar months may coincide with corresponding serial numbered solar month sometimes.. So Upakarma day fixed as "shraavaNa shukla pourNamaasya" is on the lunar month of shraavana, (the 5th month), full moon day. Corresponding 5th month is AAvaNi, so the generic usage of the name of that month for this festival.
Also, it was observed by Tamil astronomers that even though the 5th month AavaNi, a Tamil version of ShraavaNi, has full moon day on shravana nakshatra,TiruvoNam, in several years it occurred on the next star, Dhanishthaa-shravishthaa-avittam. Hence some opined that the full moon day should be referred to occur on Avittam. thus the colloquial usage as aavaNi Avittam has come into usage.
The sankalpa, in Sanskrit calls it as Adhyaaya Upaakarma. the declaration made therein is 'shraavaNyaam pourNamaasyaayaam'. So, there is no reference either to the solar month of AavaNi or the star tiruvoNam/avittam. However, whatever star whether it is Uttaraashaadha (UttiraaDam), shravaNa (tiruvonam) or dhanishtaa-shravishtaa-avittam, it is also stated to identify the actual day of performance.
Additional information:
Yajurvedis give preference to the actual scheduled time of performance of Upakarma, that is Maadhyaanhika kaala, noon time, when mahaa sankalpam and maadhyaanhika snaanam, maadhyaanhikam, and brahma yajnam with deva-rishi tarpaNam are to be performed, after which only Upaakarma can commence. At this time the PourNami tithi should have commenced.
Rigvedis give preference to Tiruvonam star and pournami tithi or at least one of the two at the time of sunrise.
For Saama vedis, it is on or a day preceding the Ganesh Chaturthi day. There are no Atharva Vedis, presently known to exist in India.
In Tamilnadu, every rite is performed according to the time prescribed. The question, what is the tithi or nakshatra today, can not be answered because 'day' is about 24 hours from sunrise to sunrise or midnight to midnight(English) and is solar, whereas tithi and nakshatra are lunar. Between sunrise and sunrise, there can be three tithis or three nakshatras even. So the question if asked should what is the tithi or nakshatra NOW and not today. Upakarma is Maadhyaahna, 1048 hours, Amavasya and shraaddha are with reference to aparaahna, 1314hours, pradosham on Trayodashi at 1715 to 1845 hours, sankata hara chaturthi at 1800 hours, Ramnavami at 1200 hours Janmashtami at 2400 hours, mahashivaratri on Maagha Krishna Chaturdashi, with reference to dusk to dawn raatrimaana, ,the rest of the above, with +/- sunrise difference from mean 0600 hours.
Answer: AavaNi avittam is a colloquial usage to just name the festival and does not identify the day as it is Not linked to solar calendar.
Source: Yahoo Answers
https://in.answers.yahoo.com/question/index?qid=20120731180216AAjYPoR