Ēkādaśī ("Eleventh"), also spelled as Ēkādaśi, is the eleventh lunar day (tithi) of each of the two lunar phases which occur in an vedic calendar month - the Shukla Pakṣa (the period of the brightening moon also known as the waxing phase) and the Kṛṣṇa Pakṣa (the period of the fading moon also known as the waning phase)..  It is according to the Vedic medical texts of Ayurveda and is mentioned in detail in many original treatises such as Charaka Samhita and Susruta Samhita.
In Nepal and India, Ekādaśī is considered a day to cleanse the body, aid repair and rejuvenation and is usually observed by partial or complete fast. High protein and carbohydrate-containing foods such as beans and grains are not consumed by observant people during the fast as it is a day to cleanse the body. Instead, only fruit, vegetables, and milk products are eaten. This period of abstinence starts from sunrise on the day of Ekādaśī to sunrise on the following day. Rice is not eaten on Ekadashi.
The timing of each Ekādaśī is according to the position of the moon. The Indian calendar marks progression from a full moon to a new moon as divided into fifteen equal arcs. Each arc measures one lunar day, called a tithi. The time it takes the moon to traverse a particular distance is the length of that lunar day. Ekādaśī refers to the 11th tithi, or lunar day. The eleventh tithi corresponds to a precise phase of the waxing and waning moon. In the bright half of the lunar month, the moon will appear roughly 3/4 full on Ekādaśī, and in the dark half of the lunar month, the moon will be about 3/4 dark on Ekādaśī.
There are usually 24 Ekādaśīs in a calendar year. Occasionally, there are two extra Ekādaśīs that happen in a leap year. Each Ekādaśī day is purported to have particular benefits that are attained by the performance of specific activities.
The table below describes the Ekādaśīs and when they fall in the year.
Ekādaśī is different for Vaishnavites and Smarthas. According to Kala Prakashika, a Jyotish text discussing auspicious times for beginning an activity ("Muhurta"), the Ekādaśī fast is performed on a day which is not touched or ruined by any influence of the tenth tithi or lunar day. The cut-off time is 96 minutes before sunrise. If the tenth day completes just 96 minutes before sunrise, then that day is celebrated as Ekādaśī. If the tenth day is incomplete at 96 minutes before sunrise, but still continues to be Dashimi sometime during that day, then the Ekādaśī fast is performed on the following day. (Rules need to be included here by a Panchang Karta from Dharma Sindhu and Nirnaya Sindhu.)
Ekādaśī Tithi, the eleventh lunar day (Shukla Ekādaśī), also known as Hari Vasara because it is dedicated to Lord Vishnu, is a day of fasting and prayers for all Hindus. Those who fast on this day are considered to get rid of malefic planetary influences, experience happiness, and gain the right peace of mind to think of Ishvara and attain moksha. It is a day of Vishtikarana, a day of malefic influences. Vishtikarana coincides with the second half of Ekādaśī Tithi and is avoided for all functions associated with worldly prosperity but for such celebrations, Ekādaśī Tithi should not have Dasami Vedha. Fasting should be done during Vishtikarana but the fast should not be broken during this. Vishtikarana coincides with the second half of Krishna Dasami.
Karana is half of a tithi. Tithi is the time taken by the moon to travel approximately twelve degrees of space with reference to the Sun, but as the motion of the moon is irregular, the duration of tithi is not constant.
There are seven moveable and four fixed karanas. Vishti or Bhadra is one of the moveable karanas which rotate among the other tithis beginning with the second half of Shukla Padyami.
The Vishnu mantra chanted on this day is: "Om Namo Bhagavate Vasudevaya"
Hare Krishna maha-mantra to chant 108 times: or Hare Rama Hare Rama Rama Rama Hare Hare, Hare Krishna Hare Krishna Krishna Krishna Hare Hare""Hare Krishna Hare Krishna Krishna Krishna Hare Hare, Hare Rama Hare Rama Rama Rama Hare Hare