Vassa, Pavarana and Kathina
The Vassa, a three-month rains retreat, was instituted by the Buddha himself and was made obligatory for all fully ordained bhikkhus; the details are laid down in the Mahavagga of the Vinaya Pitaka (3rd and 4th chapters). The retreat extends over a period corresponding to the North Indian rainy season, from the day following the full moon of July until the full-moon day of October; those who cannot enter the regular Vassa are permitted to observe the retreat for three months beginning with the day following the August full moon. From the time Buddhism was introduced to Sri Lanka by the Arahant Mahinda, the observance of Vassa -- Vas in Sinhala -- has been one of the mainstays of monastic life on the island. During the Vas the monks are expected to dwell permanently in their temples and suspend all travelling. If unavoidable circumstances necessitate travelling, they are allowed to leave their residences on the promise that they will return within a week (sattahakaraniya). On the first day of the retreat, the monks have to formally declare that they will dwell in that manner in the selected monastery or dwelling.
The Vassa is also a time for the lay Buddhists to express their devotion to the cause of Buddhism by supporting the Sangha with special diligence, which task they regard as a potent source of merit. It is customary for prominent persons to invite monks to spend the Vas with them in dwellings specially prepared for the purpose. In this latter case the host would go and invite the monk or monks formally. If the monks accept the invitation, the hosts would prepare a special temporary dwelling in a suitable place with a refectory and a shrine room. On the first day of the Vas they would go with drummers and dancers to the monastery where the invitees reside and conduct them thence in procession. The hosts would assume responsibility for providing all the needs of the monk or monks during this period, and they attend to this work quite willingly as they regard it as highly meritorious. If no special construction is put up, the lay supporters would invite the monks to observe the retreat in the temple itself.
At the close of the Vas season, the monks have to perform the pavarana ceremony. At this ceremony, held in place of the Patimokkha recitation, each monk invites his fellows to point out to him any faults he has committed during the Vas period. On any day following the day of pavarana in the period terminating with the next full-moon day, the kathina ceremony is held.
Different monasteries will hold the kathina on different days within this month, though any given monastery may hold only one kathina ceremony. The main event in this ceremony is the offering of the special robe known as the kathina-civara to the Sangha, who in turn present it to one monk who has observed the retreat. The laity traditionally offer unsewn cloth to the monks. Before the offering takes place, the robe is generally taken, with drumming, etc., around the village in the early hours of the morning. Once the robe is given to the Sangha, certain monks are selected to do the cutting, sewing, and dying of the robe -- all in a single day. Public contributions are very often solicited to buy the robe if it is not a personal offering.
This ceremony, which is performed with keen interest and devotion, has today become an important occasion of great social and religious significance for the Buddhist laity. This seems to have been so even in historical times when many Sinhala kings made this offering with much interest and devotion (e.g. Mhv. xliv,48, xci, etc).
Sources:
"Buddhist Ceremonies and Rituals of Sri Lanka", by A.G.S. Kariyawasam. Access to Insight (BCBS Edition), 1 December 2013, https://www.accesstoinsight.org/lib/authors/kariyawasam/wheel402.html
“Vassa (Rains Retreat) and Kathina (Robe Offering) Ceremony”, https://www.budsas.org/ebud/ebdha086.htm