Śrīla Bhakti Rakṣak Śrīdhar Dev-Goswāmī Mahārāj describes the observance of Dvādaśī.
Student: Mahārāj, can you tell us also about the importance of Dvādaśī?
Śrīla Śrīdhar Mahārāj: Yes. We are told, from a particular approach, that Ekādaśī and Dvādaśī are the favourites of Hari. The underlying cause of this has already been explained. So, if we serve Hari on these tithis, Hari will be pleased with even very small service. Dvādaśī is said to be even more favourite to Hari than Ekādaśī, but also a little lenient. So, we generally observe fasting on Ekādaśī, and only on the eight Mahādvādaśīs can we leave Ekādaśī and observe fasting on Dvādaśī. Otherwise, observance on Ekādaśī is compulsory. Although Dvādaśī is honoured as the very favourite of Hari, still Ekādaśī has preference for fasting. Only in the eight cases formed by a special combination of the tithi [lunar phase], the nakṣatra [lunar house], and other things does Dvādaśī get preference over Ekādaśī. At these times, our small service will give us a greater result. That is the clue, the key. On Ekādaśī and on Dvādaśī, if we render even some small service, we will get greater remuneration. Remuneration means prema [love], prīti [affection], serving attitude, earnestness. All these things will be enhanced at that particular time. The scientific cause of this is that Hari, at this time, wants to consume more service. That Hari demands more of His servitors is their fortune. So, at this time, during Ekādaśī and Dvādaśī, even a little service from His servitors is very useful. That is the key. Hare Kṛṣṇa.
Also, as Pūrṇimā and Amāvasyā approach, and the rasa, the material eccentricity, the enjoying energy, comes to its highest degree, it is checked as it begins, before the full moon and new moon, by observing Ekādaśī and Dvādaśī.
Reference
Spoken 19 February 1982.