Source: ISKCON Mangalore

Reference on Ekadashi from scriptures:The purpose of ekādaśī-vrata is that today we should not eat much usual food, grains. The actual prescription is fasting. Nirjala-ekādaśī. Nirjala means there are many devotees who does not take even water. Water, drinking water, according to śāstra, it is taking food. It is drinking of food or no food. We can take both ways. So sometimes drinking of water is excused as upavāsa also. But there are many devotees who don’t even…, drink even a drop of water. Whole day and night they fast and observe ekādaśī-vrata. And the night is called harivāsara. Harivāsara means the whole night they would chant Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma… This is called vrata. Dṛḍha-vrata. Dṛḍha-vrata. Dṛḍha means very firm, steady.But Kṛṣṇa is so kind that He has given us so many concessions. But we should be… Because there are some concessions, better we should not take advantage of the concession, but we should be very steady and strong in our conviction and perform devotional service strictly. That is nice. Vrataiḥ. Dharmato vrataiḥ. Parāvare brahmaṇi dharmato vrataiḥ snātasya [SB 1.5.7].Ref:- Swami Prabhupada Lecture on Śrīmad-Bhāgavatam 1.5.1-8, May 23, 1 969 New Vrindavan