Vaikunta Ekadashi, a fascinating Hindu temple festival
Soorga vaasal, Sriangam temple, TN Triposo
Vaikunta Ekadashi (December - January) is an important annual Hindu temple festival celebrated at all Vishu temples across India. It marks the  opening of ‘Vaikunta Dwaram’ or ‘the gate to Lord's Inner Sanctum’ through which Namperumal emerges  on the Ekadashi day, an auspicious day to bless the devotees. Vishnu Purana says fasting on the Vaikunta Ekadashi day is equal to fasting on the remaining  23 ekadashi days of both Shukla paksha and Krishna paksha. in a Hindu calendar. Hinduism emphasizes that fasting on the Ekadashi day is mandatory and a religious duty of a devout Hindu  and is believed to be away to prify ouselves.
 The area around the garbagraha / Srikovil (Sanctum) is referred to  as Vaikuntha Vaasal and devotees  walk through the vaassal (gate) to gain entry into the temple to seek  Lord Vishnu's blessing.

The Ekadashi  festival  has links with two Asuras - demons who received a boon  from Lord Vishu according to which 
whoever  patiently listens to their story with true devotion and sees the image of Vishnu emerging out of the door ( Dwar), called Vaikunth Dwar (in Tamil Soorga Vaasal or Paramapada Vaasal), for him  the gate of  Vaikuntam (heaven) will remain always open.  This is the reason why at many Vishnu Temples  there is door  called Paramapada vaasal through which people are allowed to pass  through so that they will be blessed by the God almighty. In some Vishnu temples where there is no Paramapa Vaasal, they jerry-build a Soorga Vaasal  temporarely for the benefit of the devotees.
Srirangam Temple GopuramWikipedia
Padma Purana has a different version:  The demon Muran,  unleashed atrocities on the Devas. Unable to bear  his  reign of terror, Devas frantically approached  Lord Shiva, who directed them to Vishnu. In the ensuing battle with the demon, Vishu realized the need of a new weapon to kill the mighty demon. In order to take rest and create a new weapon, Vishnu went to  a remote cave named after  goddess Haimavati in Bhadrikashrama. Upon locating Vishnu's place, demon  Muran appeared before Vishu and tried to kill him when he was fast asleep. No sooner had he raised his weapon to kill thr God, than a  female power  appeared from Vishnu, glanced at him  and  turned  Muran to ashes. This slaying of the demon happened on the eleventh day of lunar month during the sun's journey in the Dhanur Rasi. Vishnu named her as 'Ekadashi' and granted a boon that  she attained the status of a goddess - Ekadashi and  devotees who go on a fast on the Ekadashi day will be absolved of  all their sins  and reach Vaikuntham (his heavenly abode) where they will experience heavenly bliss under the shadow of the lord. Further, they are liberated  from the cycle of birth and death. So, 'Ekadashi' is symbolic of the victory over  demon Muran - the good over the evil. The first Ekadashi was a Dhanurmasa Shukla Paksha Ekadashi.

This popular temple festival brings out the three qualities of a human  mind and they are: Rajas, Tamas and Satva as revealed by Sri Krishna to Arjuna on the battle ground at Kurushetra.  People regardless of race, origin and country  have one of these mind sets or a combination of two qualities. People with Rajasic mind  experiences both good and bad qualities. In this case undharmic mind overpowers the dharmic mind. - (a mix of both bad and 
good qualities - tamasic and satva; example Ravana and Duriyodana) and Tamasic  mind is mostly confined to adharmic mind  dominated by lust, arrogance, selfishness. They are mostly cunning, ignoble and unscrupulous. (example; Sahuni of Mahabharata). Satva is confined to dhamic mind, no deviation from rightneousness  and people with satvic mind use discretion and act according to their conscience (example: Sri Rama). The demon Muran  is a god example of Rajasic mind, a combination of both Rajas and Tamas. When one conquers the mind and attains self realization, one will come closer to God. The advantage of fasting is people can keep a distance from unwanted thoughts which could be triggered by consuming  certain foods. Japa and prayer at night   merge  the mind with the self. This will instil confidence in us and will help us get rid of negative thoughts that may hamper our progress.

Vaikunta Ekadashi is celebrated on a gand scale at Srirangam Rnganathar temple, near Tiruchirapalli city, Tamil Nadu where the celebrations are held for 20 days. At Tirupati, Andhra, on the Ekadashi day, there will be unusual crowd  to seek the Lord's blessings.  It is celebrated with devotion at temples like Sri Villiputhur, Saragapani temple, Kumbakonam and Triplicane Parthasarathi temple, Chennai, Tamil Nadu and at Vishnu temples in Melcote, Mandya district, Karnataka and other places.