According to the Dharma Sindhu and Nirnaya Sindhu and other Hindu scriptures, when Dwadashi Tithi coincides with the Rohini Nakshatra from sunrise to sunset, the fast of Papanashini Mahadwadashi is observed. These scriptures don't clearly specify whether this Dwadashi falls in the Shukla Paksha or Krishna Paksha. Therefore, Drik Panchang lists Papanashini 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.
In Hindu tradition, fasting holds great spiritual significance. The scriptures describe numerous types of fasts undertaken to fulfill various desires and attain spiritual merit. Among these, the Dwadashi fasts are highly revered by followers of Hinduism.
While every Dwadashi throughout the year has its own unique importance, there are eight rare combinations known as the Ashta Mahadwadashi. These eight are Unmilini, Vanjuli, Trisparsha, Pakshavarddhini, Jaya, Vijaya, Jayanti and Papanashini. Each of these Mahadwadashi fasts is considered highly auspicious and supremely meritorious. Among them, Papanashini Mahadwadashi holds a special place, and its greatness is described in several Hindu scriptures.