Four months of rainy season - Ashaadh, Shraavan, Bhaadrapad and Aashwin are called Chaaturmaasya. During these months, two Ekaadashees - Aashaadh Krishn Ekaadashee (Dev Shayan) and Kaartik Krishn Ekaadashee (Dev Uthaan) are good to keep fast. One should take up the Vrat of Chaaturmaasya on Aashaadh Krishn Dwaadashee (12th), after Aashaadh Krishn Yoginee Ekaadashee (11th).

In the month of Shraavan, one should give up Shaak; in the month of Bhaadrapad, yoghurt; and in the month of Aashwin, milk, and in the month of Kaartik, meat. For the entire duration of Chaaturmaasya, one should observe some kind of dietary restrictions, such as cutting down on the amount of meals per day or eating only in the evening. It is also good to avoid certain foodstuffs such as eggplant, onion, garlic, beetroot, Laukee-Pumpkin and so forth. Other foodstuffs may include carrots, tomatoes, mustard oil, and varieties of cabbages, and cauliflower.

If one does not observe these restrictions for the entire Chaaturmaasya, then they may at least be observed in the month of Kaartik.
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Since during these four months Vishnu sleeps, that is why one should sleep on floor. One should not eat leafy vegetables in Shraavan; yogurt in Bhaadrapad; milk in Aashwin; and Daal (broken beans) in Kaartik. Only Shraavan, Bhaadrapad, Aashwin and Kaartik Krishn Ekaadashe are good for Grihasth people, not the other ones. All Shukla Paksh Ekaadashee should be observed."
[Padm Puraan, 5/6]