Today falls on the eleventh day of the waxing moon in Bhadrapada, a tithi the tradition calls Parsva or Parivartini Ekadashi, sacred to Lord Vishnu.
The legend, recounted by Krishna to Yudhishthira, tells how Vishnu took the dwarf form of Vamana and approached the generous demon-king Bali, asking for only as much land as could be covered in three steps.
Bali agreed despite his guru's warning, and Vamana grew to cosmic size, placing his foot on Bhuloka, his thigh in Bhuvanloka, his waist in Swargaloka, his stomach in Maharloka, his heart in Janaloka, his neck in Tapaloka and his face in Satyaloka.
With heaven and earth covered in two strides, Bali had nothing left to offer the third step but his own head, and Vishnu pressed him gently down to rule the netherworld of Patala instead.
This same day is also called Parivartini because it marks the moment when Vishnu, sleeping through the four monsoon months of Chaturmas on the cosmic serpent Sheshnag, is believed to turn over onto his other side.
The day asks devotees to fast, to remember Bali's surrender and Vishnu's mercy, and to let this turning of the sleeping Lord become a quiet turning point in their own conduct.