Chatur Mas is a period consisting of four human months and it begins from the Deva Sayana Ekadasi in Ashada month and ends on Utthana Ekadasi in Kartik Month. The four months of Shravan, Bhadrapad, Ashwina and Kartik are considered as the holiest months in the Hindu calendar. The four months is of great importance to Lord Vishnu devotees as this period is believed to be the sleeping period (Yoga Nitra) of Vishnu.
As Vishnu goes to bed on the Sukla Ekadasi of Asadha, it is called Sayana ekadasi; As He turns on His side on the Sukla ekadasi of Bhadrapada, it is called Parivartana ekadasi and when He wakes up on Sukla ekadasi of Kartika, it is called Uttana ekadasi. [A note of caution has to be sounded about His sleep. It is sleep with a trace of deep awareness. It is not like our sleep.
2) The Legendary Story:
One year of human beings is one day for the celestial counterparts (Devas and Gods). The first six months from mid Jan to mid July (Utharayana) is the day time and the next six months mid July to mid Jan (Dakshinaayana) is the night time for the Gods.
The six months of Utharayana are the daytime of gods and the months of Daksinayana are their night time. Once, the gods, who had assembled before the Lord Sri Hari on Mount Meru, took leave of Him as their night set in. Just then, the goddess of their night (Daksinayana) in the guise of a lady of dark complexion, with an axe in hand fell at the feet of god and said, “Oh merciful one! Upanayanas, marriages and other auspicious celebrations are banned during my period; I am branded as an inauspicious one and censured by all. I pray that I may be spared or I shall end my life”. The gods too pleaded on her behalf.
Lord Sri Hari was pleased to grant her the following boon:
“The first 4 months of hers; viz., Sravana, Bhadrapada, Asvina and Kartika shall be more auspicious than all the other months. Any good deeds like Japa, Vow, Gift and so on done in these months shall yield an infinitely greater merit than done in other months. Even in these four, each day is more meritorious than the previous and shall reach its climax in Kartika”.
The gods and goddess of Daksinayana were all pleased and went to their respective places. From then, the Caturmasya is hailed supreme and so we must make the best use of this period and be benefited in all ways.
3) Greastness of Chaturmasya:
The puranas and Dharma Sastras describe in detail the greatness and significance of the quadruplet of months. Among the puranas, the Varaha, the Skanda and Aditya are chief.
Asked by his consort, the goddess of earth, Sri Varaha replies as follows:
“The so-called four months are the most beloved period of mine. It is one of baths and vows, japas and homas, gifts and sacrifices. What is done in Magha is a 1000 times greater than what is done in other months, and that done in Vaisakha is a thousand times greater than that done in Magha and that done in Caturmasya is definitely greater than that done in Vaisakha”.
Reff. Skandha Puraan.