Uttarayan, also known as Uttarayanam or Uttarayana Punyakalam, is the six month period between Makara Sankranti or Makar Sankraman and Karka Sankranti (usually July 14) or Karka Sankramanam. In 2018, Uttarayan starts on January 14.
Dakshinayanam is the period between July 14 to January 14. On Makara Sankranti day, Sun transits into Makara Rashi, towards Northern celestial sphere. As per Hinduism and Puranas, on the beginning day of Uttarayana punykal Bhishma went to his heavenly abode. As per his boon, Bhishma, also known as Devavrata, had to choose a day on his own to die and to get salvation.
As per Hindu scriptures, Uttarayanam ends when Devas go for sleep. The first four months of Dakshinayan or Pitrayan are observed as Chaturmas Vratam during which Vishnu Shayana Vratam, Shiva Sayana Vratam, Dev Sayana vratam and Asoonya Sayana Vratham are observed.
Sankraman Snan or Sankramana Punya Snanam is the ritual bath performed by devotees along the river banks of holy rivers like – Ganga, Yamuna, Godavari, Krishna, Cauvery, Narmada, etc..