This is one of the eight great austerities. In the time when the Sun is in the Rishhabha rasi, that is in the month of vaikasi (mid May to mid June), on the eight moon day (ashhtami) in the growing lunar phase (shukla paxam) this vratam is observed.This vratam is described in the skandha puranam.
Getting up early in the morning, the devotee should think and salute the parameshavara with His consort uma on the bull. Later taking bath being purified, on should start the worship. Could do the worship to the Rishhabharudar form made in gold or silver. The Lord should be anointed with the substances like panychamritam and other holy things with the converging mind on the Lord. Offering to the Lord the rice and derived offerings like payasam on should pray to the God. Then to the kind hearted devotees of Lord shiva material should be donated. Worshiping the Lord in the Lord shiva temple and making offerings there should return home in that happiness. Later along with the devotees, eat the rice, sugar-rice, fruits and sweets without spoiling the intent of the austerity. Listen to the glories of Lord shiva until late evening and then sleep on the bare floor relaxedly. The following day after saluting the Lord, donate the idol to a sincere love hearted of Lord shiva.
One who observes this vratam flawlessly would get long life, wealth, wisdom, health and the eight great treasures. vishnu pleasing the paramashivam got lots of treasures and the garuda as his vehicle. Indra got the elephant called airavatam, the deva of fire got the goat, deva of death the buffalo, niruthi got human, deva of water got crocodile, deva of air got the deer, kubera got airplane pushhpaka vimanam, Ishanar got the ox that resembles the justice, sun got the chariot of one wheel and seven horses, moon got the special ruby airplane as their vehicles by observing the vrata worshiping the Rishhabharudhar. The maharishi told once to the king of maharashhtra vishvasena the things that happened, happening and would happen. The astonished king asked how the maharishhi got to know these details. The sage told him that it was because of the glory of vrishhabha vratam. The king also observed the austerity and got to know these. The king santushhta worshiping through this vratam got the eight great attainments and got the moxa. vidhrutha king roamed around with the body in the seven down worlds and the worlds of brahma, vishhnu and Indra, transpiring the knowledge of this vrata to his sons, observing this vratam for a long time, became shiva yogi. Such glorious is this vratam.
ஆத்தமாம் அயனு மாலும்
அன்றிமற் றொழிந்த தேவர்
சோத்தமெம் பெருமான் என்று
தொழுதுதோத் திரங்கள் சொல்லத்
தீர்த்தமாம் அட்ட மிமுன்
சீருடை ஏழு நாளுங்
கூத்தராய் வீதி போந்தார்
குறுக்கைவீ ரட்ட னாரே. 4.50.2