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.
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.
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