Jai Sriman Narayana !!
Dakshinayana Punyakala/Karkataka Sankrati occurs due to the changes in planetary position, Sun enters in to Cancer/Karkataka Rashi on this day, this is marked as the beginning of Dakshinayana Punyakala. During this period, Lord Vishnu undergoes Cosmic sleep (Yoganidra) in the milky ocean. The end of one Uttarayana Punyakala (which begins on Makara Sankrati day) is treated as one day for Lord. During Karkataka Sankrati /Dakshinayana Punya Kala, performing Pitru Tharpan/rituals/paying homage to forefathers and following austerities are treated as highly meritorious. Worship of Lord Vishnu on this auspicious day, giving alms and give away food and clothes are treated the most rewarding acts.