The 'Rains Retreat' or 'Rains Residence' undertaken by Theravada Buddhist monks, nuns, and laity during the summer months. Vassa is delineated by two Uposatha days, Asalha Puja and Pavarana Puja. This period, from July until October, roughly, corresponds with the rainy season in India and many south and Southeast Asian nations.
Vassa has its origins with the Buddha, who decreed that wandering monks should stay in one place during the rainy season. The reasoning behind this was two-fold. First, the inclement weather of the rainy season constituted a health hazard to monks who were not able to secure shelter. Secondly, during the rainy season plants begin new growth, and animals are in greater abundance. The Buddha wished to decrease the chances that monks would damage plants or animals in their travels by keeping them in one place. The practice of staying put for the three months of the rainy season gave rise to the system of monasteries and monastic colleges that is prominent in the Buddhist world today.
For monks, vassa marks a period of intensive practice. Since they and their teachers do not travel during the rains residence, there is great opportunity for intensive focus on the study of the Dhamma. All monks are expected to make extra efforts with their studies, whatever they may be. Meditators spend more time in meditation, scholars spend more time studying texts and discussing their interpretation. Teachers spend more time teaching.
The restriction against leaving the monastery is not absolute. Monks go out for their daily alms round to seek food, and continue to interact with the laity, giving teachings, and presiding over funerals and other significant event. Monks and nuns are permitted to go about their daily business as long as they are able to return to the place where they are passing the rains by sundown. By special dispensation, a monk may leave the monastery for up to one week, though this is exceptional
The intensity of monastic practice during vassa is intensified by an influx of new monks. During vassa, many young men, and a few young women, undertake temporary ordination, becoming a monk or nun for a specified period of time. They live the life of a Buddhist monastic during vassa, and then return to their homes. Some elect to stay in the monasteries, choosing a vocation as monk or nun. In a number of Theravada countries, particularly Thailand and (prior to being overrun by the Communists, Laos), it is quite common particularly for young men to undertake to become a monk for some period of time, and it is seen as being a great source of merit for the family, particularly his parents.
For laypersons that do not undertake to temporarily ordain, vassa can still be a time of more intensive practice, something of an elongated Uposatha. It is not unusual for a layperson to make some sort of vow during vassa, undertaking to give up some habit, or to observe some rule or religious discipline during the three month period. Vows of this form might be giving up smoking, meditating daily, observing the Uposatha, or reading from the suttas every day. Some lay followers may undertake one of the traditional austere practices set down by Buddha for monks who wished to practice in a more rigorous fashion, though most of these are not practical for a person not living as a monk.
Vassa is sometimes called, mistakenly, the Buddhist lent, since both seem to involve a practitioner giving something up. However, these two events differ in their origins and intent. The focus of vassa is not denial. Indeed, many vows undertaken by lay folk have nothing to do with giving something up, but rather with doing something extra. Vassa is a period of intensive practice and focus on the Dhamma, and not an exercise in asceticism. Lent focuses on repentance and the experiences of Christ, and in content has little in common with vassa.