Monday is dedicated to Lord Shiva. On this day, he is worshipped along with his consort goddess Parvati. Though, of course, as is the custom, Lord Ganesha is venerated right at the beginning of the worship. Devotees also listen to Shiva Bhajans on this day.
The 'Somvar Vrat' or Monday fast is observed from sunrise till sunset and the devotee partakes of the food only after saying his evening prayer. Fasting on Mondays in the month of Shravan is considered even more auspicious. Hindus believe that observing a fast or 'upvaas' on Mondays will satisfy Lord Shiva who will grant them wisdom and fulfill all their desires. In some places, unmarried girls and spinsters keep fast to be blessed with an ideal husband.
This tradition goes back to Hindu mythology, though the myths vary greatly from region to region. Among the popular ones are: There lived a very poor Brahmin who started devoting his Mondays to fasting and worshipping Lord Shiva. After a period of time, the Lord was satisfied with his devotion and blessed him with wealth. Another, lore speaks of a rich merchant who begets a son after a prolonged wait but the child dies. It is only with the grace of Lord Shiva and Parvati that the son is reborn. A third story is describes a game of dice played between Shiva and Parvati that makes Monday important for Shiva.
Color & Gem:
White is the preferred color of the day and Pearl the preferred gem.