Source - https://en.wikipedia.org/wiki/Dakshinayana
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Dakshinayana is the six-month period between Summer solstice and Winter solstice, when the sun travels towards the south on the celestial sphere. Dakshinayana begins on Karka Sankranti around July 16, as it marks the transition of the Sun into Karka rashi (Cancer).
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It marks the end of the six-month Uttarayana period of Hindu calendar and the beginning of Dakshinayana, which itself ends at Makar Sankranti and the Uttarayan period begins
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According to the Puranas, Dakshinayana marks the period when the gods and goddesses are in their celestial sleep
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Source - https://www.hindu-blog.com/2009/07/dakshinayana-punyakalam-karka.html
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Dakshinayana Punyakalam is the day when sun enters the Karkata Rashi or Karka Rasi (Cancer Zodiac) as per Hindu Astrology. It is also known as Dakshinayanam or Karka Sankraman or Karkataka Sankranti. It marks the beginning of the night of Devas. In Hinduism, a human year is equal to one day of gods and the next 6 months up to Uttarayana is night time of Gods. As per South Indian Hindu Astrology and Panchang, Dakshinayana Punyakalam begins around July 16
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In simple terms, Dakshinayana Punyakalam marks the southern transit of the Sun. The Uttarayana period which began on January 14 or 15 ends with Dakshinayana Punyakalam. The Dakshinayanam period will end on Makar Sankranti or Uttarayana day.
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Legend has it that Lord Vishnu goes into sleep during the period - the Deva Sayani Ekadasi marks this day. The Chatur Masa is also observed during this period.
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Karkataka Sankranti day is chosen by many Hindus to perform Shradh or Pinda Daan for dead relatives and ancestors. In some regions Lord Varaha, an incarnation of Lord Vishnu, is worshipped on the day.
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Source - https://www.hindu-blog.com/2008/07/kataka-kataka-sankramana.html
Kataka Sankramana, or Karkataka Sankramana, is the Dakshinayana Punya Kala. A year in Hindu calendar is divided into two halves or Ayanas. Kataka Sankramana begins around July 16 and in some places in South India, Shrarddha and Tarpana is performed on this day. One half is known as Uttarayana (January to July) and the other Dakshinayana (July to January). Uttarayana is considered to be the daytime of Devas (gods) and Dakshinayana is the nighttime. In science, Kataka Sankramana is the progress of Sun (Surya) to the south of the equator
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In North India, Karkataka Sankramana is known as Karka Sankranti. Is this period inauspicious? It is not.
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In Tamil Nadu, the Aadi month begins with Dakshinayana and no auspicious events are held as it is the first month during the nighttime of Devas. But numerous pujas including the Varalakshmi Pooja takes place in this month so there are many people who believe it is not inauspicious.
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In North, the four most auspicious months or Chatur Mas happens during Karka Sankranti period.
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Kataka Sankramana Shraddha and Tarpana are done by Hindus on this day in South India. It is a ritual dedicated to the dead family members.