SankarAnthi festival is the celebration of
UtharAyaNa PuNya Kaalam marking the travel of
the Sun to the North .In temples like
Thirukkudanathai AarAvamudhan , where the Lord rests under
Bhaaskara VimAnam , the DakshiNAyanam door is closed and
the UttharAyaNam door is opened for the next six months.
Surya Namaskara Prasnam and AruNam are source references
for the tribute to the Gathi of the Lord residing
inside the Sun as ChAyaa SuvarchalA SamEtha
Soorya NaarayaNa Parabrahmam in this Vedic tradition.
From a folksy point of view , SanakrAnthi is
the celebration of the just completed Winter
Harvest and celebration of that Subhiksham.