Literally Chaaturmaasya means four months (Chatur + Maas) and this word is derived from Sanskrit words Chatur, meaning four and Maas meaning month.

At the beginning of this period, the Sun enters Zodiac sign Cancer (Kark) and moves toward the South in the Hindu month Aashaadh. The four months of Chaaturmaas consist of Shraavan, Bhaadrapad, Aashwin and Kaarttik. The collective period of four months is considered the most suitable for the meditation and introspection.

As per the Hindu mythology, at the beginning of the Chaaturmaas Vishnu goes to Ksheer Saagar (ocean of Milk) to rest. He sleeps there for complete four months on the Shesh Naag (The Divine Snake) bed and only wakes up on Probhodini Ekaadashee - Dev Uthaan Ekaadashee, on Kaarttik Shukla Ekaadashee. It should be understood that the sleep of Vishnu signifies a phase of complete meditation and awareness. This period is not auspicious for any kind of auspicious ceremony, for example wedding etc, and other similar rituals as the gods are in their sleep during this month.

This period is perfect for meditating, listening to religious discourses and taking Holy dips in rivers. It is believed that fasting and praying during this period is very effective. One should maintain a proper vegetarian diet to avoid any diseases as the monsoons bring along a horde of ailments. This is the most beloved period of the gods and Jap done in this period bears 1000 times greater results. During the period of Chaaturmaasya, Sanyaasee avoid traveling and stay at one place. A lot of insects are produced during the monsoons and movement can trample these insects under their feet. This is the period of \enhancing one's spiritual power by meditating.

Legend Behind This Vrat
As per legend, when Vishnu incarnated as Vaaman, He stepped on King Bali's head to push him down to the Paataal Lok. After that King Bali requested Vishnu to come every year to the Paataal Lok with Lakshmee Jee. From that time Vishnu goes to his place during the Chaaturmaasya and rests there.

The King of Gods, Indra was sitting in his palace, in Heaven. Indra became very proud of himself and forgot the true source of his power. On the other hand, Bali, the son of the Asur king Virochan, vowed to purify himself of all evil and got involved in a rigorous penance. With this, the Daitya superseded the Devtaa and Indra was dethroned. Indra, with other Devtaa, soon approached and requested Vishnu to do something so that he could go back to his city. Agreeing to his request, Vishnu took the form of Vaaman, an 8 year old Braahman boy (Batuk). Shiv takes a similar form in his Batuk Bhairav Roop.

Vaaman visited Bali, who received him with great pleasure and asked Him to ask for anything. Vishnu asked for enough land to cover three steps. Looking at the little feet of the Batuk, Bali started laughing and told him to ask for a bigger thing. Vishnu was stubborn and stuck to His demand, so Bali granted the strange request. Soon, Vishnu got transformed in to a huge form, with one stride on the Earth and second in the Heavens. Laughingly He said to Bali - "I have covered everything and still have one step to go. Where shall I keep it?"

Bali soon realized who He was and immediately offered his own head for keeping the third step. Vishnu placed the third step on his head and pushed Bali back down to Rasaatal. Vishnu was very pleased with Bali and offered him a wish. Bali requested that Vishnu together with consort Lakshmee Jee should come and live with him for one third of the year - as one third of His alms was on his own head, one out of three feet land. That is why Vishnu lives there for four months. Vishnu agreed and during this time, Vishnu is considered to be asleep. Thus the start of Chaaturmaasya is called Devashayanee Ekadashee and the day He wakes  up the Chaaturmaasya ends, is called Dev Prabodhinee  Ekaadashee. Whoever hears this Kathaa (story) of the great Bhakt Bali and practice the Chaaturmaasya Vrat has the blessings of Vishnu forever.

Significance of Chaaturmaasya for a Sanyaasee
Sanyaasee perform Vyaas Poojaa as a part of Chaaturmaasya rituals. Sanyaasee observe Chaaturmaasya for 4 fortnights (two months), beginning on Full Moon day of the month of Aashaadh, also known as Guru Poornimaa, and are supposed to stay at one selected place during this period  till rains retreat, and discourse to the public. On the last day the seer crosses the border of the place of their Vrat indicating the end of the Vrat.

How to do This Vrat