
According to the Hindu Panchang, when Dwadashi Tithi coincides with the Punarvasu Nakshatra from sunrise to sunset, it is known as Jayanti Mahadwadashi. Various scriptures mention differing opinions regarding whether this Dwadashi falls in the Krishna Paksha or the Shukla Paksha. Therefore, Drik Panchang lists the day of Jayanti Mahadwadashi for both Paksha.
If Ekadashi and Dwadashi fall on the same day, one should observe the fast for both days together and perform Parana, i.e., conclude the fast on the following day. By doing so, the devotee attains the spiritual merit of observing both fasts.
However, if Ekadashi and Dwadashi occur on separate days, then ardent devotees of Lord Vishnu and those who are capable should observe fasting on both days. Those who are unable or physically weak should consume only fruits, i.e., Phalahara on Ekadashi and observe a complete fast, i.e., Nirahara Vrat on Dwadashi. Doing so does not incur any fault for not breaking the Ekadashi fast, as both fasting days are dedicated to the same Deity.
According to different opinions and the Vaishnava tradition, some devotees of Lord Vishnu skip the Ekadashi fast and observe only the Dwadashi fast, believing that the Dwadashi Vrat should never be violated. However, as stated in the Dharma Sindhu, one should not completely forsake the Ekadashi Vrat, meaning that consuming Anna, i.e., grains or any kind of food on Ekadashi is strictly prohibited even after skipping Ekadashi Vrat.
As described in the Dharma Sindhu, when Dwadashi Tithi in the Shukla Paksha coincides with the Punarvasu Nakshatra, the fast of Jayanti Mahadwadashi is observed. It is one of the eight Mahadwadashi days mentioned in Hindu scriptures. These eight Mahadwadashi are Unmilini, Vanjuli, Trisparsha, Pakshavarddhini, Jaya, Vijaya, Jayanti and Papanashini. By observing the Jayanti Mahadwadashi fast with devotion, one attains the collective merit of all other fasts.
According to the Narada Purana and Brihannaradiya Purana, when Dwadashi Tithi in the Shukla Paksha coincides with Prajapati Nakshatra, i.e., Rohini Nakshatra, it is called Jayanti Dwadashi. On this sacred day, Lord Vamana, the bestower of spiritual accomplishments, should be worshiped with devotion. Observing this fast on such a divine occasion bestows the combined merit of all fasts, the rewards of all charitable deeds and grants both material happiness and ultimate liberation.