The Gateway to Liberation
Vaikuṇṭha Ekādaśī (also called Mokṣadā Ekādaśī — the Ekādaśī that grants liberation) falls on the eleventh day of the bright fortnight of the Tamil month Mārgaḷi (December-January). It is the most celebrated of the 24 annual Ekādaśīs and draws millions of pilgrims to Śrī Raṅgam, Tirumalai, and other major Divya Deśams.
The Paramapada-vāsal
At Śrī Raṅgam, the central ceremony of Vaikuṇṭha Ekādaśī is the opening of the Paramapada-vāsal (the Door of Supreme Refuge / Door to Vaikuṇṭha) — a special gate in the inner prakarama. Passing through this gate is understood as a symbolic enactment of reaching Vaikuṇṭha. Hundreds of thousands of devotees queue through the night to pass through.
Nammāzhvār's Connection
Vaikuṇṭha Ekādaśī is also the tirunakṣatram (star-day) of Nammāzhvār. On this day at Śrī Raṅgam, after passing through the Paramapada-vāsal, the Lord Namperumāḷ 'receives' Nammāzhvār into Vaikuṇṭha — the mūrti of Nammāzhvār is ceremonially elevated into the presence of the Lord in a moving enactment of the Āzhvār's liberation.