Ekadashi

Ekadashi (Sanskrit: एकादशी, romanized: Ēkādaśī, lit. 'The eleventh day') is the eleventh lunar day (tithi) of the waxing (Shukla Pakṣa) and waning (Kṛṣṇa Pakṣa) lunar cycles in a Vedic calendar month. Ekadashi is popularly observed within Vaishnavism one of the major paths within Sanatana Dharma. Followers offer their worship to the god Vishnu by fasting or just symbolically; the idea was always to receive self-discipline and the benefits of fasting and it was connected to the way of life via Sanatana Dharma practices.
In Hinduism, the primary purpose of fasting on Ekadashi is to gain control over the mind and bodily senses, and channel it towards spiritual progression. In addition, there are several health benefits to fasting. Ekadashi fasting spans for three days. Devotees take single meal in the afternoon a day before Ekadashi day(Dasami) to make sure there is no residual food in the stomach on next day. Devotees keep a strict fast with no food or water on Ekadashi day and break the fast on next day(Dwadashi) only after sunrise. Since this form of fasting is very stringent and may not be possible for people in all walks of life due to health and life choice reasons, a more liberal from of fasting is followed by most devotees.
There are different ways to observe the Ekadashi fasting in this method, ranging from complete abstention from food and water to partial fasting or consuming only specific types of food. Devotee can chose the fasting approach that aligns with his/her health, lifestyle, and spiritual goals.
The timing of each ekadashi is according to the position of the moon. The Hindu calendar marks progression from a full moon to a new moon as divided into fifteen equal arcs of 12°. 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. Ekadashi refers to the 11th tithi, or lunar day. The eleventh tithi corresponds to a precise angle and phase of the waxing and waning moon. In the bright half of the lunar month, the moon will be exactly 3/4 full on the start of Ekadashi, and in the dark half of the lunar month, the moon will be 3/4 dark on the start of Ekadashi.
There are usually 24 Ekadashis in a calendar year. Occasionally, there are two extra Ekadashis that happen in a Hindu leap year. Each Ekadashi day is purported to have particular benefits that are attained by the performance of specific activities.
Bhagavata Purana (skandha IX, adhyaay 4) notes the observation of Ekadashi by Ambarisha, a devotee of Vishnu.
The story behind Ekadashi began with Vishnu sleeping or in a meditative state. A demon, Murdanav, approached and attempted to attack Vishnu. At that time, a beautiful woman sprung forth from Vishnu's 11th sense (often called the "mind"). An infatuated Murdanav asked to marry her, to which she responded that she would agree only if he defeated her in battle. As they fought, Murdanav was eventually killed. Vishnu awoke from his sleep, and blessed the woman by naming her "Ekadashi", and stated that if anyone were to fast on this day, they would attain moksha (liberation from the cycle of birth and death).
The table below describes the ekadashis and when they fall in the year.