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.