Upvass, or fasting, on Mondays begins at sunrise and ends at sunset. On the day, food is only eaten after evening prayer. Lord Shiva and Goddess Parvati are worshipped on the day. Of course, no worship begins without remembering Lord Ganesha. The fasting on Mondays in the Shravan month is considered even more auspicious.
It is believed that those people who observe Upvaas on Monday will have their desires fulfilled and will be blessed with wisdom. In some areas, fasting on Mondays is observed by unmarried women to get an ideal husband.
The reason for fasting on a day is usually traced back to a particular incident in one of the Hindu Puranas or folk tradition. There are numerous myths associated with a particular fast. The stories vary from region to region and from community to community.
Somvar Vrat, or Monday fasting, dedicated to Lord Shiva too has numerous stories. Among the several myths, three have gained popularity – the first is the story of a poor Brahmin gaining wealth due to Monday fasting, second is the story of a rich merchant begetting a son after long wait and the subsequent death of the son and his rebirth due to the grace of Lord Shiva and Parvati and the third story is the most famous involving the game of dice played between Shiva and Parvati.
Any Upvaas or Vrat is directly or indirectly connected with some sort of material comfort. This is a sweet lure used by ancient saints to attract a devotee to the concept of Brahman.
The concept of Brahman in Sanatana Dharma is not easily digestible to many people. And they are least bothered about Brahman concept all they need is material comfort. For this many devotees are ready to go to any extend. They are ready to observe any harsh vow. Ancient wise men used this tendency among the common people to spread the awareness of Brahman. During each upvas there are occasional flashes when a person thinks beyond material comforts. This is a realization of Brahman. Gradually many devotees begin to think beyond material comforts.
Monday also called Somvar in Hindi. In Indian culture Som is also called to Moon (Chanderma), Monday is related to Moon (Som) so it also called Somwar.
There are twelve (dwadash) jyotirlinga of Lord Shiva. First jyotirlinga name Somnath is situated at Somnath in Gujarat. At this place Som (Moon) had prayed for Lord Shiva for thousand of years then he got a worship from load shiva to stay at that place. Hence that temple is famous with the name Somnath Temple. Also Lord Shiva give his worship that the Monday (Somwar called in Hindi) will be day of Lord Shiva. For this Lord Shiva place the moon on his forehead. Also you can see moon on the Head of Lord Shiva. This is the main reason that the monday has major important for Lord Shiva.
With best regards
I do agree with Mr. Sood. Monday is Som-baar in hindi. Som means the moon. So, this day is also having importance to the Lord Moon.
The story goes this way, once the Moon, asked Lord Shiva that he too deserves to be worshipped by the people. But none of his deeds are so great that, people can consider him worth of worshipping!
The Lord Shiva, answered him that every one has contributed something for the wellness of the universe. In the appropriate time I will need your help to save the mankind. Later when Samudramanthana took place, the POISON comes from it, as they were using Basuki (king of snakes) as the rope! All the Deities and Demons get terrified by the amount of poison comes from it. They started praying Lord Shiva, to help tham and save the mankind from destruction. Lord Shiva agreed to drink all the poisons came out of Manthan but later feel verymuch uneasy. The hot sensation of poison make him restless. Then he called moon to stay with him and provide him his coolness! Moon was very much happy to be a part of this world saving act. Since than, Moon was always there in Lord Shiva's forehead. And the first day of the week is named after him also. And yes, that's how the Lord got it's other name as "Somnath".