A period of intensified practice in the life of a monastery during which uninterrupted residence is mandatory for registered monks in training. Ango means "tranquil" (an 安) "shelter" (go 居). A more formal name is "retreat in which the rules are bound" (kessei ango 結制安居). The term kessei refers either to (1) the act of "binding" (ketsu 結) a stricter "system" or set of "rules" (sei 制) of monastic training in a formal rite that marks the opening of a retreat, also called "binding the retreat" (ketsuge 結夏), or to (2) the entire period of time that the stricter rules are in force, which is also called the "period of retreat" (seichū 制中) or "during the retreat" (angochū 安居中). The end of a retreat (kaisei 解制) is marked by a rite in which the "rules" (sei 制) are "relaxed" or "loosened" (kai 解). The time between retreats (geai 解間), literally the "period" (ai 間) of "loosening" (ge 解), is when monks in training may come and register in a monastery or terminate their registration and depart. All appointments to official positions in a monastic bureaucracy are formally confirmed at the start of the retreat and remain fixed for the duration of the retreat.
At Japanese Zen monasteries today there are two annual retreats, which go by various names: (1) the rains retreat (u ango 雨安居), summer retreat (ge ango 夏安居), or summer assembly (natsu e 夏會), and (2) the snow retreat (setsu ango 雪安居), winter retreat (tō ango 冬安居), or winter assembly (fuyue 冬會). The traditional length of time for a retreat in East Asia is ninety days (kujun 九旬), or "nine" (ku九) "ten-day periods" (jun 旬), which is three months according to the Chinese lunar calendar. The dates recommended in Standard Observances of the Soto Zen School are May 15 to August 15 for the summer retreat and November 15 to February 15 for the winter retreat. These dates accord with the schedule for "middle retreats" (chūango 中安居) as established in the Soto School Constitution (Sōtōshū shūsei 曹洞宗宗制). The Constitution also allows for "early retreats" (zen ango 前安居) that begin and end one month earlier than middle retreats, and "late retreats" (go ango 後安居) that begin and end one month later.
The retreat as it is observed in Buddhist monasteries around the world today is a ritual replication of the rains retreat (S. varṣavāsa) originally observed by monks in ancient India during the three months of the monsoon. The expression "cloistered retreat" (kinsoku ango 禁足安居), still used in Japanese Zen, reflects the fact that monks were "forbidden" (kin 禁) to wander "on foot" (soku 足) during the rainy season. The Pali Canon (Mahāvagga, III, 2, 2) suggests that, at some early stage in the evolution of the Buddhist monastic order, there was a system of two dates for the assignment of seats at temporary dwelling sites set up for the rains retreat: one at the start of the rains, and a second one approximately a month later that was intended to accommodate latecomers. Subsequently, a third assignment of seats was implemented at the end of retreats, ostensibly for the purpose of reserving places for the next year's retreat, but actually to accommodate monks who planned to remain at the site for the eight or nine months of the year between retreats; that was called the "intervening" (Pali, antarā) assignment of seats. Modern scholars theorize that the phenomenon of permanent Buddhist monastic institutions evolved from that practice. Although monasteries came to be occupied on a year-round basis, the rains retreat continued to be marked by a ritual "binding" and "releasing" of the community, and the seniority of individual monks came to be reckoned by the number of annual retreats that had passed since they received the precepts and joined the order.
Monastic retreats have traditionally been understood within the Buddhist world to begin and end on the days of a full moon and last for three months, but there is much variation in their timing. Chinese sources attest to that variation in ancient India and Central Asia and evince considerable difference of opinions on the issue. The Chinese pilgrim monk Genjō (C. Xuanzang 玄奘, 600-664), for example, reported in his Record of Western Lands (J. Saiikiki 西域記) that in some countries in Central Asia the retreat ran from the 16th day of the 12th month through the 15th day of the 3rd month, because that was time of year when the rains were heavy (T 51.872a-14-15). Chinese translations of Indian Vinaya texts accurately rendered the three times for the assignment of seats for a retreat as "earlier" (zen 前), "later" (go 後) and "in between" (chū 中), but the influential Chinese Vinaya exegete Dōsen (C. Daoxuan 道宣, 596-667) seems to have misconstrued the intended meaning of those terms. In his Commentary on the Four-Part Vinaya (J. Shibun ritsu gyōji shō 四分律行事鈔) Daoxuan wrote that the "early retreat" (zen ango 前安居) begins on 4/16 and lasts for three months; the "late retreat" (go ango 後安居) begins on 5/16 and also lasts for three months; but the "middle retreat" (chū ango 中安居) begins any time from 4/17 through 5/15 and does not necessarily last for three months (T 40.38b23-26). The present day Soto interpretation of the three times for commencing retreats (early, middle, and late) derives from Dōsen's interpretation of the Indian Vinaya tradition. The Sūtra of Brahma's Net (J. Bonmōkyō 梵網經), a Chinese apocryphon that is the locus classicus for the bodhisattva precepts (bosatsukai 菩薩戒) used in East Asian Buddhism, says that disciples of the Buddha should enter into retreat for austere practice (zuda 頭陀) and sitting meditation (zazen 坐禪) twice a year, once in the winter and once in the summer (T 24.1008a13). It is the oldest source to mention such a system, which may have begun in Central Asia or China. The practice of holding two annual retreats was well established in the public monasteries of Song dynasty (960-1278) China that served as a model for Japanese Zen.