Shivratri in general is an auspicious day to worship Lord Shiva and his wife Parvati; it is celebrated only once a year. Masik Shivratri on the other hand is celebrated every month. It is a day on which many devotees of Lord Shiva pray and chant to him to attain his blessings.
History
Masik Shivratri is celebrated due to various reasons. One of the beliefs is that on this day Lord Shiva and Parvati got married while the other belief is that Lord Shiva drank a poison on this day to save the world. He drank this poison because when the heavenly deities were churning the ocean, out of this ocean came many things and one was a poison named "Samudra Manthan' which had the capability of destroying the universe. Lord Shiva saved the world by drinking it, which turned his throat blue.
Celebration
As this day is believed to be the day of the marriage of Lord Shiva and Parvati; many women hold a fast on this day for the well being of their husband and ask the blessings of Lord Shiva for a happy marriage. Unmarried woman pray on this day as well, in the hope that they will soon be blessed with a husband.
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