Varṣā

varṣā. [alt. varṣa, vārṣika] (P. vassa; T. dbyar gnas; C. anju 安居). Literally, “rains” and, by extension, “rainy season retreat.” This refers to a three-month period during which time monastics "do not go beyond the marked bounds of the monasteries and observe rules and regulations in addition to their regular routine."

Traditionally, this period begins "the day after the full-moon day of the eighth lunar month (usually July)" and concludes "on the full moon of the eleventh lunar month (usually October)."

The Princeton Dictionary states:

According to tradition, the Buddha instructed monks to cease their peregrinations during the torrential monsoon period in order to prevent the killing of insects and worms while walking on muddy roads. However, the practice of observing a rains retreat was likely adopted from other mendicant śramaṇa sects in ancient India at the time of the Buddha.