Shayani Ekadashi (lit. "sleeping eleventh") or Maha-ekadashi (lit. "The great eleventh") or Padma Ekadashi is the eleventh lunar day (Ekadashi) of the bright fortnight (Shukla paksha) of the Hindu month of Ashadha (June - July). Thus it is also known as Ashadhi Ekadashi or Ashadhi. This holy day is of special significance to Vaishnavas, followers of Hindu preserver god Vishnu.
It is believed that Vishnu falls asleep in Ksheersagar - cosmic ocean of milk - on Shesha nāga, the cosmic serpent. Thus the day is also called Dev-Shayani Ekadashi (lit. "god-sleeping eleventh") or Hari-shayani Ekadashi (lit. "Vishnu-sleeping eleventh") or Shayana Ekadashi. Vishnu finally awakens from his slumber four months later on Prabodhini Ekadashi - eleventh day of bright fortnight in the Hindu month Kartik (October-November). This period is known as Chaturmas (lit. "four months") and coincides with the rainy season. Thus, Shayani Ekadashi is the beginning of Chaturmas. Devotees start observing the Chaturmas vrata (vow) to please Vishnu on this day.
A fast is observed on Shayani Ekadashi. The fast demands abstainance from all grains, beans, cereals, certain vegetables like onions and certain spices and all kinds of meat and eggs.
Vishnu sleeps on the Shesha Shaiya - bed of Shesha.