As per Hindu mythology one human year is equivalent to one day of the Gods. And Kark Sankranti or Dakshinayana is also the period when the sun transits to the south. During this period the nights are comparatively longer and the days are shorter. In northern India Dakshinayana will start from June 21st of the year and as in southern India Dakshinayana will start from 16th of July. The month of Aadi marks the beginning of Dakshinayana in Tamil Nadu and the belief associated with it is that Lord Vishnu goes to sleep during this period. The four holy months as per Hinduism falls during the period of Dakshinayana. People who have Pitr dosh in their horoscope get the Pitr Dosh puja done during this time and those who wish peace for the souls of their ancestors perform the Shraads during this time.
The two phases of the sun's run in relation to the planet have a significant impact on the functioning of the human system. Dakshinayana is considered as the feminine phase of the earth. And this feminine phase was chosen by Lord Shiva to teach because during this phase human beings are in a receptive form.
The festivals concered with feminine energy generally starts from the beginning of the Dakshinayana. This phase is thus considered as the phase of intimacy and as the moon phase of the earth. It is said that during this phase the earth plays her role as a woman or as female gender. Moreover, the annual shraadh is generally performed during the Dakshinayana period.
In the Upanishads it is mentioned that the rituals performed during Dakshinayana period reaches the dead directly.
And in the southern India, the first Amavasi after Dakshinayana is considered as highly favorable to perform the Shraadh.
Thus, the night time of the Gods is observed as the Daksinayana or Kark Sankranti.