This is a very important day for Vaishanavas (followers of Lord Vishnu- the preserving energy of this Universe).
It is believed that Vishnu falls asleep in Ksheersagar which is the “cosmic ocean of milk” - on Shesha naga, the cosmic serpent Because of this, it is called as the Dev-Shayani Ekadashi (god-sleeping eleventh) or Hari-shayani Ekadashi (Vishnu-sleeping eleventh) or Shayana Ekadashi (sleeping eleventh).
Lord Vishnu then awakens after four months on Prabodhini Ekadashi - eleventh day of bright fortnight in the Hindu month Kartik which typically falls around the months of October–November of the Gregorian calendar.
This time of the Lord’s slumber coincides with the rainy season in India and is called as the Chaturmas (i.e. four months).
Shayani Ekadashi is the beginning of Chaturmas. On this day images of Vishnu and Lakshmi are worshipped, the entire night is spent singing prayers, and devotees keep fast and take vows on this day which they intend to observe during the entire holy “four-month period” of the rainy season.
It needs to be noted that, Chaturmas is considered inauspicious for typical worldly activities like weddings and is reserved for penance, austerities, fasting, bathing in holy rivers and other religious observances.
The Dev-Shayani Ekadashi marks the commencement of the festivities and is also the first ekadashi (eleventh) observance of the second half of the Hindu year.