The Cāturmāsya period begins in the month of Āṣāḍha (June-July) from the day of Ekādaśī called Śayanā-ekādaśī, in the fortnight of the waxing moon. The period ends in the month of Kārtika (October-November) on the Ekādaśī day known as Utthānā-ekādaśī, in the fortnight of the waxing moon. This four-month period is known as Cāturmāsya. Some Vaiṣṇavas also observe it from the full-moon day of Āṣāḍha until the full-moon day of Kārtika. That is also a period of four months. This period, calculated by the lunar months, is called Cāturmāsya, but others also observe Cāturmāsya according to the solar month from Śrāvaṇa to Kārtika. The whole period, either lunar or solar, takes place during the rainy season. Cāturmāsya should be observed by all sections of the population. It does not matter whether one is a gṛhastha or a sannyāsī. The observance is obligatory for all āśramas. 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 Śrāvaṇa one should not eat spinach, in the month of Bhādra one should not eat yogurt, and in the month of Āśvina one should not drink milk. One should not eat fish or other nonvegetarian food during the month of Kārtika. A nonvegetarian diet means fish and meat. Similarly, masūra dhal and urad dhal are also considered nonvegetarian. These two dhals contain a great amount of protein, and food rich in protein is considered nonvegetarian. On the whole, during the four-month period of Cāturmāsya one should practice giving up all food intended for sense enjoyment.
(Sri Caitanya Caritamrta----2:4:169----purport).

The Cāturmāsya ceremony is observed during the four months of the rainy season in India (approximately July, August, September and October), beginning from Śrāvaṇa. During these four months, saintly persons who are accustomed to travel from one place to another to propagate Kṛṣṇa consciousness remain at one place, usually a holy place of pilgrimage. During these times, there are certain special rules and regulations which are strictly followed. It is stated in the Skanda Purāṇa that during this period, if someone circumambulates the temple of Viṣṇu at least four times, it is understood that he has traveled all over the universe. By such circumambulation, one is understood to have seen all the holy places where the Ganges water is flowing, and by following the regulative principles of Cāturmāsya one can very quickly be raised to the platform of devotional service.
(Nectar of Devotion).

Persons who identify with bodily existence are attached to the fruitive activities described in the Vedic literature. For example, in the Vedas it is said that one who observes the cāturmāsya vow will attain eternal happiness in the heavenly kingdom.
(Srimad Bhagavatam----4:2:22----purport).

These sacrifices may be fitted into various divisions. There are persons who are sacrificing their possessions in the form of various kinds of charities. In India, the rich mercantile community or princely orders open various kinds of charitable institutions like dharma-śālā, anna-kṣetra, atithi-śālā, anāthālaya and vidyā-pīṭha. In other countries, too, there are many hospitals, old age homes and similar charitable foundations meant for distributing food, education and medical treatment free to the poor. All these charitable activities are called dravyamaya-yajña. There are others who, for higher elevation in life or for promotion to higher planets within the universe, voluntarily accept many kinds of austerities such as candrāyaṇa and cāturmāsya. These processes entail severe vows for conducting life under certain rigid rules. For example, under the cāturmāsya vow the candidate does not shave for four months during the year (July to October), he does not eat certain foods, does not eat twice in a day or does not leave home. Such sacrifice of the comforts of life is called tapomaya-yajña. There are still others who engage themselves in different kinds of mystic yogas like the Patañjali system (for merging into the existence of the Absolute), or haṭha-yoga or aṣṭāṅga-yoga (for particular perfections). And some travel to all the sanctified places of pilgrimage. All these practices are called yoga-yajña, sacrifice for a certain type of perfection in the material world. There are others who engage themselves in the studies of different Vedic literatures, specifically the Upaniṣads and Vedānta-sūtras, or the Sāṅkhya philosophy. All of these are called svādhyāya-yajña. or engagement in the sacrifice of studies. All these yogīs are faithfully engaged in different types of sacrifice and are seeking a higher status of life. Kṛṣṇa consciousness, however, is different from these because it is the direct service of the Supreme Lord. Kṛṣṇa consciousness cannot be attained by any one of the above-mentioned types of sacrifice but can be attained only by the mercy of the Lord and His bona fide devotees. Therefore, Kṛṣṇa consciousness is transcendental.
(Bhagavad-Gita----4:28----purport).