Chaturmasa spans the four Hindu calendar months of Shravan, Bhadrapad, Ashwin and Kartik. These months fall during the rainy season in India. This is when Sun starts its southwards declination (Dakshinayan).
The austere period of Chaturmasa is not only observed in Hinduism but also by the other two religions that have originated in India, namely- Buddhism and Jainism.
It is believed that the Gods and Deities are resting during this phase, because of which worldly endeavors such as weddings, thread ceremonies etc. are not performed during the Chaturmasa. During the story of Bali, Lord Vishnu promises to grace Bali with his presence and the time of Chaturmasa is when Lord Vishnu is believed to be in the patal loka (nether world).
Since these four months fall during the rainy season in India when the chances for ill-health and diseases are quite high, it is quite logical to note several of the austerities which are expected to be performed- for example, food should preferably only be eaten at home. It is not advisable to travel during this time.
Chaturmas is reserved for penance, austerities, fasting, bathing in holy rivers and religious observances. Devotees resolve to observe some form of ritualistic vow (called vrat), such as maintaining silence over an extended period or abstaining from their favorite food item or having only a single meal in a day. Several key Hindu festivals such as Krishna Janmasthami, Ganesh Utsav, Navratri and Diwali all occur during this time, allowing people to spend more time in divine prayers and reverence instead of focusing on their worldly aspirations!