Chaturmasa are four (chatur) months (masa) during which Vishnu rests on the ocean of milk, thus all waters are particularly sacred. This time of rites and vows is Chaturmasya.
It lasts from the eleventh day of the bright moon phase in Ashadha (June/July) to the eleventh day of the bright moon phase in Karttika (October/November).
The first day is Devashayani Ekadasi (Vishnu goes to rest).
The last day is Devaprabodhini Ekadasi (Vishnu awakens).