As the rainy season arrives with commencement of month of “Shravan (July-August)”, the Genuine sages and hermits who are on the way to achieve perfection in their relationship with the supreme personality of God or have already achieved perfection in same, to pause their wanderings and see them staying in huge numbers in temples and ashrams, giving opportunity to the general masses to serve the lord and get them blessed by their blessings.
During Chaturmaas , 4th July to 4 th November  , Any Auspicious Ceremonies { shush karma } , should Not be performed . Rather , Best time for Puja Upasana of Shivji , Maa Durga , Ganapati ji , Kartikey Ji, Vishnuji , Maa Mahalaxmi ji , all important festivals come in and celebrated . Vishnu ji puja is mainly done , throughout four months .
Chatumaas, the word comes from two separate words, “Chatur and Maas”. It means a collection of Four Months, Sharavan (July-August), Bhadrapada (August-September), Ashwin (September-October) and Kartik (October-November). These four months are specifically chosen since it involves the complete rainy season and the autumn season. These seasons, the sages stay at a particular place and perform austerities of body, speech and mind.
As per Ayurveda, these four months are described as the months of Highest Weakness in the body since the Sun changes it’s course, “Daksinayan”. Surya becomes more prominent in the Southern Hemisphere and the withdrawal causes generalized weakness. And since the weather is moist and humid, the body is depleted of many nutrients via bodily secretions.
“The Caturmasya period begins in the month of Ashadha (June-July) from the day of Ekadasi called Sayana-ekadasi, in the fortnight of the waxing moon. The period ends in the month of Kartika (October-November) on the Ekadasi day known as Utthana-Ekadasi, in the fortnight of the waxing moon. This four-month period is known as “Caturmaas”. 
Some Vaishnavas also observe it from the full-moon day of Ashadha until the full-moon day of Kartika. That is also a period of four months. This period, calculated by the lunar months, is called “Caturmaas”, but others also observe Caturmaas according to the solar month from Sravana to Kartika. 
The whole period, either lunar or solar, takes place during the rainy season. 
Caturmasya should be observed by all sections of the population. It does not matter whether one is a grihastha or a sannyasi. The observance is obligatory for all asramas. The real purpose behind the vow taken during these four months is to minimize the quantity of sense gratification. This is not very difficult. 
In these months, our shastras say , that people must restrain from few things in  their Daily Routine Diet.
Scientific Paradigm
The rainy season, carries along with itself a plethora of pathogens and insects who move out from their residence inside the ground in order to find shelter somewhere else as their homes are flooded with water.
Also, due to cool weather, humid climate and availability of water makes the pathogens grow in an accelerated manner. These pathogens become the cause of diseases and hence, to avoid the contact with pathogens, one must observe cleanliness and must restrict movement.
Certain food items must not be consumed :
Green Leafy Vegetables In Shravan Month
Yogurt In Bhadrapada Month
Milk In Ashwin Month
Non-Veg, Urad And Masoor Dal In Kartik Month
Vedic and Scientific  reason for the same is:
In Shravan month, due to fresh arrival of monsoon, all the small organisms living inside the ground come out and try to find shelter. Green leafy vegetables being the best in nutrition as well as space give them the best opportunity. They may enter the leafy vegetables and hence it is recommended to avoid leafy vegetables .
In Bhadrapada, the season becomes too much cold and wet. Hence, taking yogurt will cause respiratory diseases.
In Ashwin, the mammals whose milk we drink have been enough poisoned by eating the leafy vegetables. For two months, they have been doing so and hence, their milk becomes indigestible and we have the weakest strength at that time due to high humidity and clear sky.
In Kartik, the body starts to recover and welcome the winter season. Since the season is about to change and dryness is to come, consuming foods which are dry and take long time to digest is not considered wise.
One should not eat fish or other non vegetarian food during Chaturmaas , especially the month of Kartika. A non vegetarian diet means fish and meat etc. Similarly, masur daal and urad daal also may be avoided .
On the whole, during the four-month period of Caturmaas one should practice giving up all food intended for sense enjoyment.
Urja Vrata – means restricting eating.The term “spinach”  refers to green leafy vegetables of that nature.The yogurt fast is fasting from eating yogurt by itself. Milk fast means refrain from drinking milk. Milk sweets, milk by-products are okay.
In the fourth (last) month, Ashvin Shukla Dashami to Kartik Shukla Dwadashi one should avoid taking dicotyledon grains (grains breaking in two parts) like gram, tur urad, masur etc., Similarly, one should not eat fruits or vegetables containing lot of seeds.  Thus reason for making a list of prohibited items separately for each of the four months is in the nature of dietary discipline, and declared as daily regimen “Nityavrata” by the scriptures.
Also during the Purushottam Adiik Maasa there is no observance of chaturmaas.
Chaturmasya means “four months”, which is the duration when Vishnuji Sleeps , few main festivals are :
Devshayani Ekadasi, Asadha 11 (July 4 2017). The time VishnuJi goes to sleep for four months.
Shrawan Maas 10th july to 7th August , 2017
Kaamika Ekadashi 19/20 july 2017
Shrawan Amavsya 23 july 2017
Hariyali Teej 26th july 2017
Naag Panchami, Shravan  (27 july 2017). 
Rakshabandhan 7th August 2017
Janmashstami 14TH / 15TH August
Somvati Amavsya 21st August
Surya Grahan 21st August
Ganesh Chaturti 25th August
Anant Chaturdashi , Ganapati Visarjan 5th September
Pitra Paksh Bigins 6th September 
Sarwa Pitra Amavasya Puja Tarpan 20th September 
Ashwin Navratri Bigins 21st September
Durga Ashtami Saraswati puja 28th September 
Maha Navmi 29 th September 
Dusseshra 30 th September
Rama Ekadashi 15th October
Govatsa Dwadashi 16th October 
Dhanteras 17 th October
Narak Chaturdashi 18th October 
Deepawali 19th October
Govardhan Puja 20th October
Bhaiya Duj , Chandra Darshan 21 st October
Devutthan Ekadashi , 31 st October
Tulsi ji Vivaha 1st November
Kartik Purnima , Guru Purab , 4th November 2017
Chatur Maas Ends “Vishnuji Wakes” , New Moon In