Saphala Ekadashi is the Ekadashi of the dark half of Pausha (Margashirsha in the amanta reckoning), deep in winter, and its katha is about Lumpaka, a prince thrown out for his wild ways. Without meaning to, he kept this Ekadashi under an old peepal tree, hungry and awake from the cold, and the fast made his life fruitful, which is what Saphala means.
For the next Saphala Ekadashi and its Smarta and Vaishnava days, see the Ekadashi dates page; each day's page there gives the parana window for your city. Here is the katha.
The katha
Yudhishthira said, "Krishna, tell me about the Ekadashi of the dark half of Pausha. What is its name, and which god is worshipped on it?"
Krishna said, "Out of love for you, I will tell you. It is called Saphala. As the serpent Shesha is first among serpents and Garuda among birds, Ekadashi is first among fasts. On Saphala, worship Narayana with fruits. Listen to its story."
In the city of Champavati ruled King Mahishmata. He had four sons, and the eldest, Lumpaka, was a great trouble to him. Lumpaka gambled, drank, chased after other men's wives, and spent his father's wealth as if it were water. He mocked the gods and insulted holy men. Every day someone came to the palace with a fresh complaint.
At last the king's patience ran out. He called his son before the court and said, "You have shamed this house long enough. Leave my kingdom." His relatives and servants agreed, and Lumpaka was sent out with nothing.
He went into the forest nearby. By day he hunted and ate what he killed; by night he crept into his father's city to steal. The people knew him, but out of respect for the king they did not harm him.
In the forest stood a very old peepal tree, which the people of the area honoured as a holy place where God was felt to dwell. Lumpaka made his shelter at its foot, not out of devotion but because it was dry and wide.
It was the month of Pausha, and on the night of the dark Dashami a bitter cold came down. Lumpaka had no blanket and hardly any clothes. He shivered all night, unable to sleep, his teeth chattering, until he lay half-frozen and could not move.
The sun rose on Ekadashi, but he did not stir until it was well up in the sky. By midday he was able to sit up, but he was too weak to hunt. He stumbled around the trees and gathered fruits that had fallen to the ground. By the time he came back to the peepal, the sun was setting.
He was too exhausted to eat. He put the fruits down at the root of the tree and said, almost without thinking, "Let Lord Vishnu be pleased with these fruits." Then, cold again, he sat awake through the second night as well.
So without knowing it, Lumpaka had fasted on Ekadashi, offered fruit to Vishnu and kept the night vigil. Vishnu accepted all of it.
In the morning a voice came from the sky: "Prince, by the merit of Saphala Ekadashi your faults are cleared. Go back to your father. Your kingdom will be yours." His heart changed in that moment. He was ashamed of how he had lived, and grateful. He walked home, bowed at his father's feet and asked forgiveness, and the king, seeing a different man before him, embraced him and in time handed him the kingdom.
Lumpaka ruled with care, worshipped Vishnu and kept every Ekadashi. He had a son named Manojna, and when the boy was grown he gave him the throne and spent his last years in devotion.
Krishna said, "Whoever keeps Saphala Ekadashi with faith gains merit greater than great sacrifices. Whoever hears its story receives its blessing."
Reading the story calmly
Lumpaka is exiled by his own father and nearly dies of cold. Neither is held up as something to copy. The story is about a person whom everyone has given up on, and about the smallest turning, a handful of fruit and a few words, being enough for God to meet him.
What the day is kept for
Saphala is kept so that effort may bear fruit: for work that has not yet paid off, for a child who has gone astray, or for one's own fresh start. The text says the fast gives success here and liberation hereafter; families keep it in the spirit of the story, with hope rather than calculation.
How it is kept
Fruit is at the centre of the day: Vishnu is offered seasonal fruits, and in many families the fast is a fruit fast. A ghee lamp is lit, and some people light lamps at a peepal tree in the evening, remembering Lumpaka. Giving a blanket or warm clothes to someone in need is a custom in some homes, since the story turns on a freezing night. The full order is on the Ekadashi puja vidhi page.
Neighbouring Ekadashis
The Ekadashi before this one is Mokshada Ekadashi, the day of the Gita, and the one after is Pausha Putrada Ekadashi, kept for children. For the tithi on any day in your city, see the panchang.