According to Dharma Sindhu, Nirnaya Sindhu and other sacred texts, when the Dwadashi Tithi extends in duration, the Dwadashi that remains from sunrise to sunset, i.e. first one, is known as Vanjuli Mahadwadashi. For example -
| 10 Dashami | 11 Ekadashi | (12) Vanjuli | 12 Extended Dwadashi |
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, fasts observed on Dwadashi Tithi are considered highly auspicious. Various sacred scriptures mention numerous meritorious fasts to be performed on this day. Among them, the Vanjuli Mahadwadashi Vrat holds great significance and is one of the eight sacred Mahadwadashi. It is also known as Vanjulika Dwadashi.
As described in the Brihannaradiya Purana and Narada Purana, when Dashami overlaps Ekadashi at sunrise and Dwadashi extends in duration, one should skip observing Ekadashi and instead perform the Vanjuli Mahadwadashi Vrat. On the day of Vanjulika Dwadashi, Lord Sankarshana should be worshiped with devotion, offering sandal paste, flowers and other sacred items. Observing this fast with faith and sincerity grants the devotee, the state of Abhaya Pada, i.e., freedom from fear. This Mahadwadashi is believed to bestow the combined merits of all Yajnas, destroy all sins and grant every form of prosperity.