What is "dakshinayana"

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 or July 16, as it marks the transition of the Sun into Karka rashi ( Cancer).

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.