This fast is observed on Jyeshtha Shukla Ekadashi.
One has to abstain even from water from sunrise of Ekadashi to sunrise of Dwadashi and this fast is known as Nirjali Ekadashi. During the month of Jyestha the day is longer and one feels thirsty. As a result it is difficult to observe this vow.
Vyas Muni told Bhimsen to observe this fast on Jyestha Shukla Ekadashi. By observing this fast on Jyestha Shukla Ekadashi sins of eating on all prior ekadashis in the year will be destroyed and absolved, and you will get all the benefits normal fasting on all the Ekadashis , which have passed in the year. Bhim was told so by Vyasa. Bhimsen very sincerely observed this fast, that is the reason why this Ekadashi is also named as Bhimsen Ekadashi.