Varuthini Ekadashi is the dark-fortnight Ekadashi that comes as spring becomes summer. The word varuthini refers to armour, and the day is kept for protection, for the sense of being sheltered from harm. It is kept in particular in Gujarat and Maharashtra, and by pilgrims on the way to the sacred rivers.
Varuthini Ekadashi falls in the Krishna paksha, the waning fortnight, of Chaitra in the amanta calendar (months that end at the new Moon). In the purnimanta calendar of north India, where the month ends at the full Moon, the same fortnight belongs to Vaishakha.
The katha
The tradition attributes the stories of the Ekadashis, Varuthini Ekadashi among them, to the Padma Purana and the Bhavishyottara Purana, where they are told as conversations, often with Krishna answering Yudhishthira. The story of Varuthini features a famous king in the middle of his austerities.
King Mandhata, a ruler of the Ikshvaku line known for his generosity, had given up his throne for a time to perform penance in a forest by the river Narmada. One day, as he sat in meditation, a bear came up and began to attack him, biting his foot and dragging him through the undergrowth. The king did not strike back, for he was observing a vow of non-violence, and prayed instead to Vishnu.
Vishnu appeared and, with his discus, killed the bear. But Mandhata was in pain, having lost his foot. Vishnu told him not to be disheartened, and said that the injury was the result of an old action, and that he should go to Mathura, worship the Varaha form of Vishnu, and keep the Varuthini Ekadashi with faith.
The tradition says the king did exactly this and that his limb was restored, so that he returned to the throne whole. The tale is told to show a mind that stays calm under attack, and to say that the vrat offers support to those who are in difficulty.
How the fast is kept
Observance follows the general pattern: fast from grain, worship Vishnu, and spend the day in a quiet way. Because the story names the Varaha form of Vishnu, some families worship an image of Varaha, the boar avatar.
In the story, the king keeps a vow of non-violence, and some devotees take the day as a reminder to restrain anger, to speak gently and to avoid harm to any creature.
Reading the tables for Varuthini Ekadashi: the date is the day on which Ekadashi is present at local sunrise, as Smarta practice has it. Some Vaishnava calendars would move Varuthini Ekadashi by a day, and you should follow yours if so. The parana follows at dawn on the next day, between sunrise and the close of Dwadashi, never during the first quarter of Dwadashi, called Hari Vasara. Nobody needs to harm their health for Varuthini Ekadashi. If a full fast is not wise for you, a simple meal of fruit and milk or a quiet day of prayer keeps the spirit of it, and your doctor should guide you on medicine and diet.
Where it sits in the year
In the round of the year, Kamada Ekadashi comes before Varuthini Ekadashi and Mohini Ekadashi after it. The Ekadashi page lists all the named Ekadashis. For Varuthini Ekadashi the practical steps are in the Ekadashi vrat vidhi, and the meaning of the tithi is on the Ekadashi tithi page.