from the Brahmanda Purana
When: During the waning moon in the month of May/June
The topics of Apara Ekadasi are described in the Brahmanda Purana in the conversation between Lord Krsna and Maharaja Yudhisthira.
Once Maharaja Yudisthira asked of Lord Krsna, "O Lord Krsna, O Janardana, what is the name of the Ekadasi that occurs during the waning moon in the month of May/June and what are its glories? Please describe all this to Me"
Lord Krsna replied, "O Maharaja Yudhisthira, you have asked Me an intelligent question that is actually beneficial for everyone.
The name of this Ekadasi is Apara. O king! This Ekadasi awards a tremendous amount of piety to those who observe it and destroys all their sinful reactions. By observing this Apara Ekadasi many grave sinful activities are destroyed such as killing a brahmana, killing a cow, killing an embryo through abortion, criticizing others, illicit sex life, speaking lies, giving false witness, bragging to others, reciting or teaching the Vedas for the sake of money, and concocting one's own scripture.
A cheater, a pseudo astrologer, and a cheating physician are also sinful as those who give false witness. All these sinful activities are totally eradicated by observing this Apara Ekadasi. Any ksatriya who gives up his profession and runs away from the battlefield certainly falls down from his occupational duties and goes to hell. If such a person observes this Ekadasi with faith he also attains the heavenly planet.
Lord Krsna continued, O King! A disciple who after receiving knowledge from his spiritual master engages in blaspheming his spiritual master certainly accumulates great sins. Such a condemned sinful person can be relieved from his sinful reactions by following the Apara Ekadasi and can attain the supreme destination.
O king of Kings! The result one obtains by taking one accumulates by taking bath at Prayaga in the month of January when the sun enters Capricorn, the results one obtains by observing the vow of Sivarati at Kasi,and by offering oblations at the lotus feet of Visnu at Gaya, and the auspicious results one obtains by taking bath in the river Gautami when the planet Jupiter enters leo, by visiting Kedarnatha during Kumba-mela, by visiting and worshipping Badrinatha, by taking bath at Kuruksetra during a solare eclipse, and by giving elephants, horses, cows, gold and land in charity are all obtained easily by observing the vow of Apara Ekadasi.
It is like the sharp axe for cutting down the tree of sinful activities to ashes. It is like the brilliant sun for dissipating the darkness born of sinful activities and like the lion to a deer in the forest of sinful activities. O King! By observing this Apara Ekadasi and worshipping Lord Visnu in His form as Trivikram, a person attains the all-auspicious abode of Lord Visnu.
Anyone who hears or reads the glories of this Ekadasi, that i have described to you for the benefit of everyone, is relieved of all his sinful activities."