“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 Caturmasya. 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 Caturmasya, but others also observe Caturmasya 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 the month of Sravana one should not eat spinach, in the month of Bhadra one should not eat yogurt, and in the month of Asvina one should not drink milk. One should not eat fish or other nonvegetarian food during the month of Kartika. A nonvegetarian diet means fish and meat. Similarly, masura dal and urad dal are also considered nonvegetarian. These two dals contain a great amount of protein, and food rich in protein is considered nonvegetarian. On the whole, during the four-month period of Caturmasya one should practice giving up all food intended for sense enjoyment.
(A.C. Bhaktivedanta Swami Prabhupada. Sri Chaitanya Charitamrita. Madhya-lila 4.169. purport.)