Yes , the the term Sadasiti is explained in Padama Purana - Vol. 1 - Shrishti Kanda - Chapter 77- Shloka 44.
44 We (i.e. I) shall , O Sage, (now) tell (you) all the fruit that
is (obtained by one) on these (samkranti days) . (The samkranti days)
on which the sun enters the Dhanu ...