Āvani (Shravani) between mid of August and September: Month of legacy
Sun's annual transit over Leo is termed as Āvani (based on the sidereal solar calendar). Āvani, the term comes from the nakshatras Shravana and Shravishta (another name of Dhanishta). Shravana and Dhanishta are paired nakshatras like the pairs Punarvasu and Pushya, Purva and Uttara Phalguni etc. This month carries the combined energy of both the above said nakshatras thus the name Shravani that became Āvani in Tamil. Shravani is also the name of Goddess Lakshmi, the Goddess of eternal prosperity. Whatever you start at Shravana gets its final stage at Shravishta, that's the philosophy. For example, God Vishnu after conquering the three worlds through his Vamana Avatar (deity of Shravana), gave it to the Ashta Vasus/Digbalas, the eight directional deities of Shravishta to take care of it for eternity. So it represents the inquisitive, technical, long term action plan that stands for ages through time, aka a legacy.
This month carries the energy of what i said above. As in what you were thinking and working on last month will get fine tuned this month, at the same time what you start this month carries the energy of achieving perfection over a long term. The full Moon day of this month falls on Shravishta nakshatra is the most potent part of this month.
"Even people with the name Shravani and people with strong presence of Shravishta carry the same energy of this month (esp. if Sun and Moon are in this axis)."
Sun moves south from the Tropic of Cancer towards the Tropic of Capricorn hence this month is the time of dusk, as in the time period comes after sunset.
Religious and spiritual significance
It is done on the full Moon day of this month when the Moon is transiting Dhanishta nakshatra. Upanayana is a religious rite conducted by chanting special vedic mantras, conducting yagyas and let the student wear a Janai (जनै/பூநூல்) for the first time so that they can enter the vedic Gurukul for the first time. Upakarma means continuing the rites every year by changing their Janai (जनै/பூநூல்). Upakarma is also performed for clearing the silly mistakes we do everytime we perform the poojas around a year.
“Puranas say that when God Vishnu took the avatar of Vamana, Lord Surya performed Upanayana before he started his vedic education.”
Varalakshmi Vrat pooja is usually conducted in the month of Adi (previous month) but some people perform it in this month also.