Somewhere there was a discussion regarding sunya / soonya thithi.
Usually if there are two thithis in the same month the first thithi
is treated as Athika thithi / soonya / shunya thithi and the second thithi
is normal thithi. Whenever Sun Transits from one sign to another, it is
the beginning of a new Hindu month under Sourava Mana masa (solar month) system.
This shankranti is treated as Shunya thithi when no shrarddha is performed.
Instead next / previous thithi in the same solar month can be used for
shrarddha if it is not shunya thithi.
