In this answer there is verse, which has word "Shadashiti Mukheshu" which has beem interpreted as Sankramana of Mithuna, Kanya, Dhanu and Meena.
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 , Mithuna , Mina and Kanya (signs of zodiac) are called Sadasiti.
Sadasiti (षडशती) is one of the name of the four passages of the sun from one zodiacal sign to the other. The Padma Purana explanins that Sadasiti. is the period of passages of the sun in Zodiac signs Sagittarius , Gemini , Pisces , Virgo. i.e. when the sun enters in these Zodiac signs that transition or Sankranti is called as Sadasiti sankranti.
Similarely when the sun enters in Zodiac signs Taurus , Scorpius , Leo , Aquarius it is called as "Vishnupadi" and when in Aries, Libra it is called as "Vishuvat".