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.