One of the name of Lord shiva is soma - saha uma. For the Lord someshvara Who wears the soma, the moon crescent on the matted hair, on the somavara day (Monday), this festival is observed.
Though all Mondays can be observed as vrata there are speacial mondays when it is much more emphasised to observe like the shrAvaNa somvAr and kArthikai sOmavAram. According to skandha purANa, on the Mondays of the month kArthikai (mid Nov to mid Dec) this vrata is observed.
Getting up at the dawn one meditates on the Supreme shiva. After taking bath and having done the morning twilight salutation, praying the Auspicious and Graceful Lord shiva, worships the couples who are devotees of the Lord, chaste and loveful as the Divines couple pArvati-paramEshawara themselves. Saluting those couples, offering them feast, the observer of this vratam eats only one time food that day. If unable to do the worship, then one would go to the temple and salute and pray the umA-maheshwara. Anointing the Lord with panychAmR^itam and other pleasent substances, hailing the Lord offering bilva leaves, not being money-minded offering the nice food materials to the Lord, sharing the food with the kind hearted devotees one eats the food. One may fast also completely.
One who observes this vratam would get to enjoy many pleasures in this world and finally would reach the kailAsham. The moon observed this vratam and got his "moonness". He saluted the Lord gauri- shankara and requested that this vratam be called soma vAra vratam (One of the names of the moon is soma. One of the name of Lord shiva with goddess umA is also soma) and whoever follows this vratam should get their wishes materialised by the grace of the Lord and finally should get the liberation. God granted the boon.
Long ago a princess called sIman^thini was observing this vratam regularly. Then two young boys cunningly came as if they were a married couple. The princess worshiped them. The young boys really became male and female couple ! The austerous sage vaShiShTa got the chaste arun^thadhi as his wife. One gets blessed by the shiva-shakti on observing this vratam sincerely.
பைங்கண்வாள் அரவணை யவனொடு பனிமல ரோனுங்காணா அங்கணா அருளென அவரவர் முறைமுறை யிறைஞ்சநின்றார் சங்கநான் மறையவர் நிறைதர அரிவையர் ஆடல்பேணத் திங்கள்நாள் விழமல்கு திருநெல்வே லியுறை செல்வர்தாமே. 3.92.9
1. Somavara vrada puja - in Tamil