Answer
Vow and fasting are not the same
In speaking of Ekadashi, we use two different words — vrata (vow) and upavasa (fasting). They are not interchangeable. A vrata is a sacred discipline taken up with a pious resolve. The Lord, for instance, calls celibacy a vrata in the Gita:
“With a tranquil mind, free from fear, established in the vow of brahmacharya, controlling the mind and fixing it on Me, the disciplined yogi should sit, devoted to Me.”
There is no fasting involved in the vow of celibacy. The vow is a moral and spiritual discipline. A 'satya-vrati' is one who has resolved to speak only the truth. The 'eka-patni vrata' is another such resolve — Sri Rama held it so firmly that even after Sita left the world, he did not marry again, although polygamy was not considered adharma for kings of that era.
Ekadashi as a vrata
Seen in this light, Ekadashi is primarily a vrata. One who adopts it as a vow should observe it continuously and according to discipline until it is formally concluded through the uddyapan (completion rite).
Those who wish to take up the Ekadashi vrata are best advised to first observe a few Ekadashis informally, to see whether body and mind are ready. Only when clarity is reached should it be formally adopted, after which the rules prescribed in the main Puranas should be followed. As this is a Vaishnava vrata, it is not appropriate for those who eat meat. Those who otherwise consume garlic and onion should also avoid them on Ekadashi.
Ekadashi without formal vrata
Those who observe Ekadashi informally should not expect worldly desires to be fulfilled through it. Even so, devotion to Lord Vishnu deepens gradually — itself a significant gain. The scripturally prescribed twice-a-month full or partial fast is also useful for the body, helping with detoxification.
Fasting as tapas
The literal meaning of tapas is 'to heat' or 'to discipline through restraint'. In a single-day fast, body and mind are disciplined through controlled hunger.
There are two kinds of tapas. One builds endurance, develops self-discipline and purifies consciousness — Ekadashi belongs to this category. The other consists of fasts and austerities that injure the self. The Gita calls these tamasic:
“That austerity which is performed with foolish stubbornness, causing pain to oneself or intended to harm others, is declared to be tamasic.”
The Puranas recommend one meal on Ekadashi, so there is no self-torture in it; it is a sattvic, traditional form of tapas.
The Gita's larger redefinition of tapas
The Gita does not place fasting at the centre of tapas. It divides tapas into three: of the body, of speech, and of the mind.
- Body — worship of the divine, the learned, teachers and the wise; purity, straightforwardness, brahmacharya and non-violence (17.14).
- Speech — speech that does not disturb others, that is truthful, pleasant and beneficial, together with regular svadhyaya (17.15).
- Mind — serene cheerfulness, gentleness, silence, self-control and purity of feeling (17.16).
What real tapas looks like
Observe Ekadashi — but do not turn it into self-punishment. God is of the nature of sat-chit-ananda, and you are a part of that same nature. He does not delight in seeing you in pain.
If you truly want to practise tapas, speak gently to others and study the Gita, the Ramkatha and the Bhagavata Purana regularly. The purity of mind that grows from such study is greater than the purity produced by fasting alone. Alongside Ekadashi, practise the austerities of speech, mind and body, and hold the inner cheerfulness — manah-prasada — which the Gita ranks as a higher form of tapas.
Through daily, contemplative study of the Gita, desires gradually weaken, attachments loosen, and suffering begins to lessen. Remaining connected to sat-chit-ananda, divine joy naturally blossoms in the mind. This is the true meaning of vrata, upavasa and tapas.