The Ekādaśī, why we observe? To become pure. Brahmacarya tapasya, austerity, penance, celibacy, keeping the mind always in Kṛṣṇa consciousness, keeping the body always cleansed—these things will help us to keep us in goodness.
Śrīmad-Bhāgavatam 7.9.8 -- Seattle, October 21, 1968
Just like Ekādaśī day—there is no eating sumptuously. Simply you take little fruits and flowers. Try to avoid that also. You don't take even water. That is really ekādaśī. But because we cannot do it—in the Kali-yuga the time is different—therefore we are allowed to take little fruit and milk, which is called anukalpa. These are different methods of tapasya.
Pandal Lecture -- Bombay, April 7, 1971
Tamāla Kṛṣṇa: Sometimes we have heard that ekādaśī is a inauspicious alignment of the planets and therefore one has to counteract this inauspiciousness by more chanting.
Prabhupāda: (laughs) No, no. It is the most auspicious. And chanting is more effective. [break] Yes.
Morning Walk -- July 5, 1975, Chicago