Source - https://www.drikpanchang.com/ceremony/upakarma/yajurveda/yajurveda-upakarma-date-time.html
Upakarma means beginning or Arambham and it refers to the ritualistic beginning of learning Veda. Apart from learning Vedas on Upakarma day, Brahmins ritually change their Upanayana thread along with Shrauta rituals. Upakarma which is a Vedic ritual is still practiced by Hindus of Brahmin community
Those who follow Yajurveda observe the Upakarma on the full moon day during Shravan month i.e. on Shravana Purnima day. Those who follow Rigveda observe the Upakarma on Shravana Nakshatra day in the month of Shravana. Hence the day to observe Upakarma might differ for the followers of Yajurveda and Rigveda
Upakarma is known as Avani Avittam in Tamil Nadu. For those who do their first Upakarma, it is known as Thalai Avani Avittam. In Andhra Pradesh, Upakarma during Shravana Purnima is known as Jandhyala Purnima
Brahmins choose the day of Shravana Purnima to begin the study of Vedas as the day coincides with Hayagriva Jayanti. Hayagriva Jayanti is the birth anniversary of Lord Hayagriva who is considered the incarnation of Lord Vishnu. It is believed that Lord Hayagriva restored all Vedas to Brahma
The main purpose of the Upakarma ritual is to offer prayers and express the gratitude to those Rishis who gave the knowledge of Vedas and revealed Vedic Mantras to the mankind. The next day of Upakarma is known as Gayatri Japam day.
Those who follow Samaveda observe the Upakarma on Hasta Nakshatra day during Bhadrapada month. Samaveda Upakarma day usually falls after fortnight of Yajurveda and Rigveda Upakarma day
Source - https://en.wikipedia.org/wiki/Upakarma
Upakarma (Sanskrit) "Beginning", also called Avani Avittam (Tamil) and Janivarada Hunnime (Kannada),Gamha Purnima (Odia), is a Vedic ritual practiced by Hindus of the Brahmin caste. This ritual is also practiced by the Kshatriya and Vaishya community, who are dvijas and therefore have the rights to do Sandhyavandanam, the daily ablution ritual.
Upakarma is conducted once a year during the shravana or Dhanishta nakshatra of the Hindu calendrical month Sravana, when Brahmins ritually change their upanayana thread accompanied by relevant srauta rituals, making sraddha offerings to the rishis, whom Hindus believe composed the Vedic hymns. The day, also called Sravana Purnima "Full Moon of Sravana" in other parts of India, usually occurs the day after the Sravana nakshatra, which also marks the Onam festival of Kerala.
On the following day, usually coinciding with the Raksha Bandhan festival in Northern and Central India, the Gayatri Mantra is recited 1008 times.
Brahmins belonging to the Samaveda do not perform upakarma rituals or change their thread on this day but rather on Bhadrapada tritiya, the third day of the month Bhadrapada with Hastaa nakshatra. Shukla Yajurvedic Brahmins of North India and Odisha do upakarma the previous day if the full moon spans two days.