Vedic Rules for Observing Ekadasi Fast on the Dvadasi Day

In our Vaisnava calendar we carefully follow all the required Vedic rules for calculating the Ekadasi Fasting dates so that the Ekadasi observer receives proper and sufficient merits from the fast.

1) If Punarvasu Naksatra falls on the Dvadasi day of bright fortnight (waxing moon), then this Fast is called Jaya Mahadvadasi.
2) If Sravana Naksatra falls on the Dvadasi day of bright part (waxing moon), then Ekadasi fast is performed on the Dvadasi day and it is called Vijaya Mahadvadasi.
3) If Rohini Naksatra falls on the Dvadasi day of bright part (waxing moon), then Ekadsi fast is done on the Dvadasi day and it is known as Jayanti Mahadvadasi.
4) If Puksa Naksatra falls on the Dvadasi day of the bright part (waxing moon), then Ekadasi fast is done on the Dvadasi day instead of on the Ekadasi day, and it is named Papanasini Mahadvadasi day.
5) If Ekadasi remains at the day break on the Dvadasi day, then it is called Unmilini Mahadvadasi day, and the Ekadasi fast is performed on the Dvadasi day.
6) If ninety minutes before sunrise Ekadasi remains and Dvadasi remains the whole day and mixes with Trayodasi on the next morning, then fasting is done on Dvadasi day and it is called Vyanjuli Mahadvadasi fast.
7) On any bright part of a month (waxing moon), if Ekadasi tithi is in the very early morning and Trayodasi remains the whole day till the end of that night, and Dvadasi tithi may or may not be seen (even a fraction of Dvadasi), it is called Trisprusha Mahadvadasi, and the Ekadasi fast is performed on the Dvadasi day.
8) If Amavasya (new moon) or Purnima (full moon) tithis extend till the next day, then Fasting for Ekadasi is performed on the Dvadasi day in that fortnight.

[Hari Bhakti Vilasa/Brahmanda Purana, sourced by Sri Krsna Balaram Swamiji, KBS0117]