The four Yugadi tithis mark the beginning of each of the four cosmic Yugas: Krita (Satya), Treta, Dwapara, and Kali. Per the Vishnu Purana, as cited in Smriti Muktaphala (Kala Kanda): “Vaisakha Shukla Tritiya, Karthika Shukla Navami, Proushtapada (Bhadrapada) Krishna Trayodashi, Magha Amavasya: these are the four Yugadis.” They are observed as Shraddha (ancestor-offering / tarpanam) days.