The story
Viṣṇu's heaven is Vaikuṇṭha, and its northern gate is held to stand open for this one day. Temples that have such a gate — the Paramapada Vāsal at Śrīraṅgam, the Vaikuṇṭha Dvāram at Tirumala — open it before dawn and let the queue pass through.
The Padma Purāṇa gives the day a personification: a maiden born of Viṣṇu who defeated the demon Murā while he slept, and was granted that whoever fasts on her day is freed from what they have done. Her name is Ekādaśī, and every eleventh day of every fortnight belongs to her; this is the one the texts call the greatest.
Telugu speakers call it Mukkoṭi Ekādaśī, the ekādaśī of the thirty million, because the gods are said to come down to earth that day. In Kerala the great day is Guruvāyūr Ekādaśī at the Kṛṣṇa temple there.
