Nirjala Ekadashi Brief Legend
Legend has it that Bhimsen, the second among the Pandava brothers, was a voracious eater. He always had a feeling of guilt because all the members of his family - his four brothers, his mother and his wife - were observing all the ekadashi vrats without fail but he was not able to observe an ekadashi vrat because of his uncontrollable hunger.
With a view to find a solution, he once asked Ved Vyas who was also the grandfather of Pandavas, for help. He said to Ved Vyasa that his elder brother, Yudhishthir, always asked him to observe ekadashi, but he was not able to do one, and asked him to show him some way out. He said that he can pray, do charity, observe all the rules, but he found himself unable to fast. He said that if there is no way out, he is ready to fast on an ekadashi, instead of on all 24 ekadashis.
Sage Ved Vyasa suggested him to fast on Nirjala ekadashi, the ekadashi which falls in the shukla paksha in the Jyestha month; during this period the Sun is present in either in Tauras or Gemini rashi. He should not eat or drink anything for 24 hours on this day. Observing this ekadashi will give him the benefit of observing all the 24 ekadashis.
Since then, Bhim started observing this ekadashi to get the maximum benefit of the vrat. Devotees follow his footstep now.
Legend behind Ekadashi: Ekadashi is the goddess that born from the eleven indriyas (ekadash) of the God to kill the demon Mura. Pleased with the divine act of her, Lord Vishnu blessed her that anyone who observes fast on her birthday will be freed from all his or her sins and will attain Moksha.