The Uposatha (Sanskrit: Upavasatha) is a Buddhist day of observance, in existence from the Buddha's time (600 BCE), and still being kept today in Buddhist countries. The Buddha taught that the Uposatha day is for "the cleansing of the defiled mind," resulting in inner calm and joy. On this day, both lay and ordained members of the sangha intensify their practice, deepen their knowledge and express communal commitment through millennia-old acts of lay-monastic reciprocity. On these days, the lay followers make a conscious effort to keep the Five Precepts or (as the tradition suggests) the eight precepts. It is a day for practicing the Buddha's teachings and meditation.
Depending on the culture and time period, uposatha days have been observed from two to six days each lunar month.
In general, Uposatha is observed about once a week in Theravada countries in accordance with the four lunar phases: the new moon, the full moon, and the two quarter moons in between. In some communities, such as in Sri Lanka, only the new moon and full moon are observed as uposatha days.
In Burmese Buddhism, Uposatha (called ဥပုသ်နေ့ ubot nei) is observed by more pious Buddhists on the following days: waxing moon (လဆန်း la hsan), full moon (လပြည့်နေ့ la pyei nei), waning moon (လဆုတ် la hsote), and new moon (လကွယ်နေ့ la kwe nei). The most common days of observance are the full moon and the new moon. In precolonial Burma, Uposatha was a legal holiday that was observed primarily in urban areas, where secular activities like business transactions came to a halt. However, since colonial rule, Sunday has replaced Uposatha as the legal day of rest. All major Burmese Buddhist holidays occur on Uposathas, namely Thingyan, the beginning of Vassa (beginning in the full moon of Waso, around July, to the full moon of Thadingyut, around October). During this period, Uposatha is more commonly observed by Buddhists than during the rest of the year. During Uposatha days, Buddhist monks at each monastery assemble and recite the patimokkha, a concise compilation of the Vinaya.
In Mahayana countries that use the Chinese calendar, the Uposatha days are observed ten times a month, on the 1st, 8th, 14th, 15th, 18th, 23rd, 24th and final three days of each lunar month. Alternatively, one can only observe Uposatha days six times a month; on the 8th, 14th, 15th, 23rd and final two days of each lunar month. In Japan, these six days are known as the roku sainichi (六斎日, Six Days of Fasting).
The word Uposatha derives from the Muluposatha Sutta (AN 3.70), in which a lay woman Visakha, goes to the Blessed One and says she is oberserving the Uposatha day. The Buddha replies that there are different Uposatha days, then proceeds to tell her the correct version of the Uposatha day. The Uposatha of the Noble Disciples.
On each uposatha day, devout Upāsaka and Upāsikā practice the Eight Precepts, perhaps echoing the Buddha's teaching that laypeople should "imitate" arhats on Uposatha days. The first five of the eight precepts are similar to the five precepts, that is, to refrain from killing living beings, stealing, wrong speech and to abstain from intoxicating drink or drugs, but the third precept is abstinence of all sexual activity instead of refraining from sexual offenses. The eight precepts are similar to the ten precepts observed by novice monks, except that the seventh and eighth precepts for the novices are combined, the ninth novice precept becomes the eighth, and the tenth novice precept (non-acceptance of gold and silver, use of money) is excluded as being impracticable for a lay person. Thus, the final three precepts are to abstain from eating at the wrong time (after midday); to abstain from entertainment such as dancing, singing, music, watching shows, as well as to abstain from wearing garlands, perfumes, cosmetics, and personal adornments; and to abstain from luxurious seats and beds.
For lay practitioners who live near a Buddhist temple, the uposatha is an opportunity for them to visit it, make offerings, listen to sermons by monks and participate in meditation sessions. For lay practitioners unable to participate in the events of a local monastery, the uposatha is a time to intensify one's own meditation and Dhamma practice, for instance, meditating an extra session or for a longer time, reading or chanting special Buddhist texts, recollecting or giving in some special way.
The eight precepts are meant to give lay people an impression of what it means to live as a monastic, and the precepts "may function as the thin end of a wedge for attracting some to monastic life." The objective of the eight precepts is different from the five in that they are less moral in nature, but more focused on developing meditative concentration, and preventing distractions. Among the eight precepts, the third precept is about maintaining chastity. Buddhist tradition therefore requires lay people to be chaste on observance days, which is similar to the historical Indian tradition of being chaste on parvan days. As for the sixth rule, this means not having food after midday, in imitation of a nearly identical rule for monks. Fluids are allowed. Taiwanese physician Ming-Jun Hung and his co-authors have analyzed early and medieval Chinese Buddhist Texts and argue that the main purposes of the half-day fast is to lessen desire, improve fitness and strength, and decrease sleepiness. Historically, Chinese Buddhists have interpreted the eight precepts as including vegetarianism.
The seventh precept is sometimes also interpreted to mean not wearing colorful clothes, which has led to a tradition for people to wear plain white when observing the eight precepts. This does not necessarily mean, however, that a Buddhist devotee dressed in white is observing the eight precepts all the time. As for the eighth precept, not sitting or sleeping on luxurious seats or beds, this usually comes down to sleeping on a mat on the floor. Though not specified in the precepts themselves, in Thailand and China, people observing the precepts usually stay in the temple overnight. This is to prevent temptations at home which break the eight precepts, and helps foster the community effort in upholding the precepts.
On the new-moon and full-moon uposatha, in monasteries where there are four or more bhikkhus, the local Sangha will recite the Patimokkha. Before the recitation starts, the monks will confess any violations of the disciplinary rules to another monk or to the Sangha. Depending on the speed of the Patimokkha chanter (one of the monks), the recitation may take from 30 minutes to over an hour. Depending on the monastery, lay people may or may not be allowed to attend.
Describing his experience of Uposatha day in Thailand, Khantipalo (1982a) writes:
Early in the morning lay people give almsfood to the bhikkhus who may be walking on almsround, invited to a layman's house, or the lay people may take the food to the monastery. Usually lay people do not eat before serving their food to the bhikkhus and they may eat only once that day.... Before the meal the laity request the Eight Precepts [from the bhikkhus] ..., which they promise to undertake for a day and night. It is usual for lay people to go to the local monastery and to spend all day and night there.... [In monasteries where] there is more study, [lay people] will hear as many as three or four discourses on Dhamma delivered by senior bhikkhus and they will have books to read and perhaps classes on Abhidhamma to attend.... In a meditation monastery ..., most of their time will be spent mindfully employed – walking and seated meditation with some time given to helping the bhikkhus with their daily duties. So the whole of this day and night (and enthusiastic lay people restrict their sleep) is given over to Dhamma.
Anniversary of the assembling of 1250 monks in the Buddha's presence during which time he delivered the "Ovada-Patimokkha Gatha."
In Tibet and Bhutan, there are four full moon Uposatha days that are of importance
In China, Japan, Korea and Vietnam there are certain full moon Uposatha days of importance.