“The Cäturmäsya period begins in the month of Äñäòha (June-July) from the day of Ekädaçé called Çayanä-ekädaçé, in the fortnight of the waxing moon. The period ends in the month of Kärtika (October-November) on the Ekädaçé day known as Utthänä-ekädaçé, in the fortnight of the waxing moon. This four-month period is known as Cäturmäsya. Some Vaiñëavas also observe it from the full-moon day of Äñäòha until the full-moon day of Kärtika. That is also a period of four months. This period, calculated by the lunar months, is called Cäturmäsya, but others also observe Cäturmäsya according to the solar month from Çrävaëa to Kärtika. The whole period, either lunar or solar, takes place during the rainy season. Cäturmäsya should be observed by all sections of the population. It does not matter whether one is a gåhastha or a sannyäsé. The observance is obligatory for all äçramas. 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 the month of Çrävaëa one should not eat spinach, in the month of Bhädra one should not eat yogurt, and in the month of Äçvina one should not drink milk. One should not eat fish or other nonvegetarian food during the month of Kärtika. A nonvegetarian diet means fish and meat. Similarly, masüra däl and urad däl are also considered nonvegetarian. These two däls contain a great amount of protein, and food rich in protein is considered nonvegetarian. On the whole, during the four-month period of Cäturmäsya one should practice giving up all food intended for sense enjoyment.
(A.C. Bhaktivedanta Swami Prabhupada. Sri Chaitanya Charitamrita. Madhya-lila 4.169. purport.)