Today is Dec 20th and Vaikunta ekadasi as per Indian calendar. A very auspicious day. It is said that the doors of Vaikuntam (The abode of Lord Vishnu) are open for any one today, and that the souls parting their bodies on this day will proceed straight to Vaikutam.

Traditionally Hindus from all walks of life observe fast till dawn, visit a Vishnu temple and have either fruits or Phala aharam after sighting moon and stars (Ancestors). The effect of fasting on Ekadasi itself is immense, I think I will need another blog for that. There are so many interesting stories related to Ekadasi vrata mahima. One great example is story of King Ambarisha (One of the kings in Raghuvamsam) who could withstand the wrath of Sage Durvasa. The story is very interesting.

King Ambarisha was a very dutiful and pious king, he was famous for his valour, righteousness and his bhakti towards Lord Vishnu. He was so famous that there were praises about his devotion to lord Vishnu even in heaven. Praises lead to envy, envy lead to anger and providence of Ambarisha made him the candidate for test of devotion. Demigods incited Sage Durvasa to test his devotion to Lord. The sage is known for his anger and curse. Sage visited Ambarisha on an Ekadashi day. The king was more than happy to receive an athidhi on the auspicious day. He washed the sacred feet of sage and invited him to join him for dinner. Sage gladly accepted but had intentions of his own. The sage replied that he had to finish his evening rituals and had to go to river bank to finish the same. The King agreed and said he will wait for the sage to return and then break his fast by sage's company.

So left the sage, no signs of return and Ekadashi was ending. Crest fallen king inquired how to get out of this situation. To get the fruit of Ekadashi (i.e, to please lord) he had to break his fast, to break his fast he had to eat without the sage (in those days having food before the guest could have been considered a great insult to Athidhi deva). Learned pundits and scholars suggest that he can take a sip of water to break his fast which is not considered as having food.
As the King was just about to sip, Sage entered the scene flared with anger, blinded by his tapashakti he created a "Kruthya" a demon to devour the king.

The king could stand and fight the demon but he thought it is not just to raise a weapon over sage like Durvasa, so he prayed to Lord Vishnu to protect him if he was right in his actions. Lo and behold , there came the mighty Sudharshana (the very discus weapon of Lord) spitting fire as it spinned through the air with razor sharp edge, in a flash it ripped out the demon into two and started moving towards sage, all his tapashakti couldn't stop the weapon. His powers gone, ego admonished he fell at Kings feet, King said he cannot order a lords servant (i.e, Sudharshana), no where to go the sage took to heels and went to all gods, Indra , Shiva, and Brahma. None could stop the flare of the disc, finally Sage reached Vaikutam and fell at lords feet and sought forgiveness for insulting his devotees and trying to disturb some one doing Ekadashi Vratam. The ever graceful lord forgave him and withdrew his discus. Such is the power of Ekadashi vratam.

In particular, this day is more important since this is most loved day by Lord himself. In Mahabaratha, Bhagavad Gita - the conversation between Lord Krishna and Arjuna at the beginning of Kurukshetra War is said to have occurred on this day about 5000 years ago. All Vishnu temples will open the eastern door or the uttara dwara, as it is said that today Lord Vishnu is facing east.

You can do a little live search and find the legends behind the Vaikuntha Ekadasi and read more about importance of this day.

Below are some quick links :
http://www.hindu-blog.com/2007/12/gita-jayanti-december-2007.html
http://en.wikipedia.org/wiki/Vaikunta_Ekadasi