One of the most well-known and popular days of religious observances in the lunar calendar of the Hindus, is the ekādaśī. It is the eleventh day from the full-moon or the new-moon, each month. It is considered as a ‘vrata’ or a religious observance, characterised mainly by fasting.
(See EKĀDAŚĪ for details.)
Out of the several ekādaśīs considered as even more sacred, Bhīmaikādaśī is one. According to the Matsyapurāṇa. (69.19-65) Śrī Kṛṣṇa is said to have imparted the knowledge of the Bhīma-dvādaṣī vrata including those rites to be performed on the daśamī (the tenth) and the ekādaśī (the eleventh) days to Bhīma, the second of the Pāṇḍava princes. Hence the names ‘Bhimaikādaśī’ and ‘Bhīmadvādaśī’. They are observed on the eleventh and the twelfth of the bright fortnight of the month of Māgha (February), with or without the conjunction of the star Puṣya. The latter day used to be called ‘Kalyāṇinī’ also.
On the tenth day (Māgha śukla daśamī) the votary is expected to take a bath after applying ghee to his body and worship Viṣṇu with the mantra ‘Oṁ namo nārāyaṇāya’ followed by the worship of Garuḍa, Śiva and Gaṇeśa. On the eleventh day (ekādaśī) a total fast is to be observed. On the next day (dvādaśī) he has to bathe in a river, raise a maṇḍapa or a pavilion in front of his house, hang a jar full of water but with a hole at the bottom, receive the drops of water on his palm repeating the name of God throughout the night and perform homa (offering oblations into a duly consecrated fire) the next morning with the help of twelve priests learned in the Vedas, honouring them suitably at the end. On the thirteenth day (trayodaśī), 13 cows have to be donated. He should then listen to the itihāsas and purāṇas.
According to some versions, this vrata was taught by the sage Pulastya to the king Bhīma who was the father of Damayantī, the wife of Nala.