Pap means sin, mochani means relief, so papmochani is translated as “Relief from your sins”.
This is observed during the waning phase of moon in the last month of the year(April is the beginning of the new year, it was later changed by the English people to January). This particular Ekadashi is said to be the last one among the 24 Ekadashi fasts.

You can redeem all your sins by observing fast on this day and it is dedicated to Lord Vishnu!

The importance of this fast was revealed to Yudhishtira by God Krishna and is given in the Bhavishyottara Purana.

As per the legend an Apsara named Manju Ghosha lured a sage named Medhavi in to married life. After years of marriage life, the Apsara told the Sage that she wanted to return back to Devaloka but the Sage in a fury refused. The Apsara disobeyed the Sages words, the angry sage cursed the Apsara and changed her in to a witch and told her that she would be relieved only if she observes Papmochani Ekadashi.