The first day after the full-moon of Āsāḷhā month (around July every year) is the first day of the three month “Rains retreat” or “Buddhist lent” for every Theravāda Buddhist monk and nun. They are required to observe vassāna or ‘Rains Retreat’ for a duration of 3 months every year, which lasts from July/August to October/November. This time corresponds to the monsoon season in South and Southeast Asia (which were historically Buddhist strongholds). During that time traveling was (and still is now) very difficult and often dangerous. Monks and nuns were often invited by Buddhist communities to reside in village ārāmas (monasteries and nunneries) for the duration of the ‘rains’, where they could fully dedicate themselves to the practice for three full months.
The tradition of ‘Rains Retreat’ was started by the Buddha Himself in the year 588 BCE, where the Lord and 60 bhikkhus resided in the vicinity of Sarnath near Vārāṇasī. During those 3 months, the venerables learned and practised Dhamma intensively under the direct guidance of the Buddha; at the end of which, all 60 Bhikkhus (monks) had attained the highest spiritual fruit of arahantship.
At the conclusion of that inaugural ‘Rains Retreat’, the Buddha instructed the Arahant elders to:
“Go forth O bhikkhus, for the welfare and happiness of the many, out of compassion for the world; propagate the Dhamma…. excellent in the beginning, middle and end….in letter as well as in spirit… There are beings with little dust in their eyes, who without hearing the Dhamma will falter and fall away… There will be those who understand Dhamma.”
Monks and nuns would ideally spend their vassāna period engaging in intensive meditation and other spiritual pursuits. The Buddha, seeing that the period of intense spiritual practice and communal discipline had greatly benefitted the monks, formally instituted the ‘Rains Retreat’ whereby monks and nuns were to refrain from traveling and engage in any tiring activity, in order to focus their attention inwardly towards spiritual development and purification.
At the end of the rough-weathered ‘Rains Retreat’, many of the monks’ robes were often close to tatters; hence the Buddha also allowed monks and nuns to receive offerings of new pieces of clothes from lay devotees to be cut, sewn and dyed into new robes. This allowance has today evolved into the much-celebrated ‘Kaṭhina’ Ceremony marking the end of vassāna.
In the Theravāda School of Buddhism, the tradition of observing the vassāna persists till today. In fact, the seniority of a Theravāda bhikkhu and bhikkhunī is reckoned by the number of ‘Rains Retreat’ they have observed since their ‘upasampadā’ (higher ordination).