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.