Amavasya is a monthly festival that comes 12 times in a year, in case of Adhik Masa it can also happen 13 times in a year.

The new moon that falls on Monday is called Somvati Amavasya. According to the probability theory of mathematics, Amavasya can occur on Monday only once or twice in a year. But according to the time cycle, it is absolutely uncertain that the new moon falls on Somvati.

During Haridwar Kumbh, the day of Somvati Amavasya is considered very holy, on this day the Naga Sadhus also take a royal bath.

According to the beliefs of Hinduism, Somvati Amavasya has a special significance. There are some different beliefs related to the worship of Somvati Amavasya.

Shivratri Vrat is a monthly fasting festival that comes 12/13 times a year, hence this fast is also called Masik Shivratri. Which comes on the Chaturdashi day of Krishna Paksha before Amavasya. Out of the monthly Shivratris, two are most famous, Falgun Triyodashi is famous as Maha Shivratri and the other is known as Sawan Shivratri. This festival is dedicated to Lord Shiva-Parvati, on this day devotees offer Belpatra on Shivling, the symbol of Lord Shiva.

The reason for Pradosh Vrat is that the Trayodashi Tithi of the month falls during Pradosh Kaal. Pradosh Kaal starts 45 minutes before sunset and ends 45 minutes after sunset.

Shani Pradosh Vrat 

When the Pradosh day falls on Monday on a weekly day, it is called Som Pradosh, Pradosh falling on Tuesday is called Bhaum Pradosh and Pradosh falling on Saturday is called Shani Pradosh.

Though Trayodashi Tithi is the best for the worship of Lord Shiva, it is even more beneficial to worship Shiva during Pradosh.