This four-month period is known as Chaturmasya. Some Vaishnavas also observe it from the full-moon day of June-July until the full-moon day of Kartika. That is also a period of four months. This period of Chaturmasya is calculated by the lunar months. Chaturmasya period is observed during the rainy season and it is observed by all groups of people.
The real purpose behind the vow taken during these four months is to minimize the quantity of sense gratification. This is not very difficult. During the Chaturmasya period, one should not eat spinach, yogurt and should not drink milk. One cannot eat fish or any non vegetarian food during the month of Kartika. Masur dal and urad dal are also considered non vegetarian so these are also not consumed during Chaturmasya period. On the whole, during the four-month period of Chaturmasya one should practice giving up all food intended for sense enjoyment.