Amalaki Ekadashi is the only Ekadashi that is kept in honour of a tree. Amalaki is the Sanskrit name of the amla, the Indian gooseberry, and the tradition says Vishnu is present in the tree on the day. It falls in Phalguna, a few days before Holi, and in Varanasi it is also called Rangbhari Ekadashi, the day on which the colours of Holi are first played.
Amalaki Ekadashi falls in the Shukla paksha, the waxing fortnight, of Phalguna. Because the bright half of a month is named alike in the amanta and purnimanta systems, north and south agree on the month of Amalaki Ekadashi.
The katha
Stories of the kind told for Amalaki Ekadashi are attributed by the tradition to the Padma Purana and the Bhavishyottara Purana, usually as a talk between Krishna and Yudhishthira, though the speakers vary by telling. The katha is told about a king and a hunter.
The king of Vaidisha, in the version told by the sage Vasishtha, ruled a kingdom where everyone, from ministers to farmers, kept Amalaki Ekadashi and worshipped the amla tree. Once a hunter wandered into the forest and, tired and hungry, came upon a temple near an amla tree where villagers were keeping the Ekadashi vigil.
Having nowhere else to go, he sat with them through the night, listening to the stories and the singing, and he ate nothing, for he had nothing. In the morning he returned home without having caught any animal. Soon after, the hunter died.
The tradition says that because of the Ekadashi he had unintentionally kept, he was reborn as a prince named Vasuratha and became the ruler of a large kingdom. The story is told to say that being present at a vigil, with a mind turned to Vishnu, can change the course of a life. It also explains why the amla tree is worshipped as a form of Vishnu on this day.
How the fast is kept
The first step is to bathe and then go to an amla tree if one is nearby. The tree is watered, wound with a thread, offered flowers, turmeric and a lamp, and circled. Where there is no tree, some families keep a branch or a few fruits of the amla before the image of Vishnu.
Amla fruit is eaten on the day in many homes, as it is rich in vitamin C and is valued in Ayurveda. In Varanasi the day is when the town begins Holi, with a procession of Shiva and the first colours. See also Holi.
The dates for Amalaki Ekadashi below come from the Smarta rule, the sunrise test. Your sampradaya may ask for a different day, since Vaishnava calendars decline an Ekadashi that is mixed with Dashami before dawn, and in that case follow them. The fast of Amalaki Ekadashi is opened on the following morning, in the parana window given in the Timing column, which lies after sunrise and inside Dwadashi and skips Hari Vasara. Fasting on Amalaki Ekadashi is a personal discipline, not a test. If you are pregnant, nursing, elderly, diabetic or on regular medicine, ask your doctor what is safe, and know that a light fast of fruit and milk, or a day simply kept in prayer, is a full observance.
Where it sits in the year
In the round of the year, Vijaya Ekadashi comes before Amalaki Ekadashi and Papmochani Ekadashi after it. The Ekadashi page lists all the named Ekadashis. For Amalaki Ekadashi the practical steps are in the Ekadashi vrat vidhi, and the meaning of the tithi is on the Ekadashi tithi page.