Ekadasi means 'the eleventh' in Sanskrit and refers to the eleventh day of a fortnight belonging to a lunar month.
Falling in the moon's waxing phase in the month of Dhanurmasa (December-January), Ekadasi is synonymous with fasting and abstinence.
The Dhanurmasa sukla paksha ekadasi is called Vaikunta Ekadasi. However, the austerity is associated with much deeper aspects.
This is explained in the Padma Puranam (the second amongst the eighteen puranas), which tells us about the beginnings of Ekadasi with this legend.
During the Krita Yuga, also termed the golden age, an asura called Muran harassed both devas and mortals.
Unable to bear his tyranny, the devas sought refuge in Shiva. He directed them to approach Vishnu.
Vishnu agreed to help the devas and went out to battle with Muran. During the long battle, which lasted one thousand celestial years, Vishnu needed to rest for a while.
He entered a beautiful cave called Himavati in Badarikashrama to sleep. Muran wanted to strike Vishnu while he was sleeping.
However, Shakti - Vishnu's female energy - emerged out of his body and assumed the form of a beautiful damsel who fought Muran and vanquished him.
When Vishnu awoke he was very pleased and named this maiden as Ekadasi and granted her a boon.
The maiden said, "O Lord, if You are pleased with me and wish to give me a boon, then give me the power to deliver people from the greatest sins if they fast of this day".
Vishnu granted her the boon and declared that people worshipping her would reach Vaikunta.
Thus, it is said, was born the first ekadasi, which was a Dhanurmasa sukla paksha ekadasi.
Sages and devout people have been since, observing the ekadasi vrata regularly.
Here, the demon Muran represents the tamasic and rajasic qualities in us. The import of the ekadasi vrata is that one can conquer rajasic and tamasic
tendencies in us through fasting. This helps us reach our ultimate destination, Vaikuntha, which is the place or stage of no 'kuntitha' or dullness, stupidity and misery.

It is also said that on this day the ksheerasagara manthana (churning of the ocean) took place and the nectar of immortality (amritha) was distributed to the gods.
This divine event is interpreted in the following manner.
The ocean of milk is the human heart filled with satvic tendencies (kindness, purity and goodness).
However, in all of us there exist both types of inclinations, good (devas) and bad (asuras).
Only Sadhana or the churning process will throw out the poison in us and we can procure the sweet nectar, which will liberate us from the cycle of birth and death.
Shaivaites call this day as Nanjunda Ekadasi, as it was on this day Shiva consumed nanju, the poison that emanated from the churning of the ocean.

Some schools of thought believe that Sri Krishna gave the Gitopadesam to Arjuna on the day of Vaikunta Ekadasi.
For the Vaishnavas, it is a very holy day and should be ideally spent in fasting, prayer and meditation.
Devotees look upon this austerity as a means to succeed in their spiritual endeavours.
To them it is an activity which will increase their sankalpa (that which is beneficial to them),
so as to avoid activities which are not beneficial to them (vikalpa). Such austerities make a student of spirituality enthusiastic, positive, serene and determined. They finally lead to the understanding that all activities must be only performed for the pleasure of the Lord.
Austerity is indeed one of the four legs supporting religion, the other three being purity, mercy and truthfulness.