Uposatha
An Uposatha () day is a Buddhist day of observance, in existence since the Buddha's time (600 BCE), and still being kept today by Buddhist practitioners. 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 ten precepts. It is a day for practicing the Buddha's teachings and meditation. Observance days

Depending on the culture and time period, uposatha days have been observed from two to six days each lunar month. Theravada countries

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  ) is observed by more pious Buddhists on the following days: waxing moon ( ), full moon ( ), waning moon ( ), and new moon ( ). 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. Mahayana countries

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 . Names of full moon Uposatha days 
The Pali names of the uposatha days are based on the Sanskrit names of the nakśatra (Pali: nakkhatta), the constellations or lunar mansions through which the moon passes within a lunar month. History 
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 observing 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. Practice

Lay practice 

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. Presently, the uposatha vows are mostly associated with Theravāda Buddhism in South and Southeast Asia, but it was a widespread practice in China as well, and is still practiced. 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. Monastic practice 

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. Communal reciprocity 

Describing his experience of Uposatha days in Thailand, Khantipalo (1982a) writes:

Special Uposatha days 

In Thailand five full-moon Uposatha days are of special significance and are called puja:
 Visakha Puja or Visakha Uposatha  or Vesak ("Buddha Day") is the most sacred Buddhist holiday. It is the anniversary of the Buddha's birth, awakening and parinibbana. Asalha Puja or Asalha Uposatha ("Dhamma Day") is the anniversary of the Buddha's delivering his first discourse, which is collected as the Dhammacakkappavattana Sutta. The three-month-long Vassa retreat starts the following day. Pavarana is the end of the Rains Retreat residence during which time each monk atones before the Sangha for any offense they may have committed. Anapanasati Day is the anniversary of the Buddha's delivering the Anapanasati Sutta. This event is not connected to an Uposatha (Poya) day in Sri Lanka and perhaps is particular to Thailand. Magha Puja or Magha Uposatha ("Sangha Day") is the anniversary of the assembling of 1250 monks in the Buddha's presence during which time he delivered the "Ovada-Patimokkha Gatha." In Sri Lanka, three full moon Uposatha or Poya days are of special significance. Vesak Poya, which is described above. Poson Poya corresponds to the Jeṭṭhā uposatha, which falls in June. It is of special significance in Sri Lanka because the monk Mahinda, Asoka's son, officially introduced Buddhism to Sri Lanka on this day in the 3rd century B.C. Esala Poya corresponds to Āsāḷhā uposatha, the full moon of July, and is described above. This day has special significance in Sri Lanka because it was the day that 56 nobles, headed by Prince Ariṭṭha, became the first Sri Lankans to be fully ordained as a bhikkhus at Cetiyagiri in Mihintale by Mahinda and his companions. It therefore marks the founding of the Lankan Bhikkhu Sangha. In Tibet and Bhutan, there are four full moon Uposatha days that are of importance
 Chotrul Duchen 
 Saga Dawa Duchen
 Chokhor Duchen
 Lhabab Duchen

In China, Japan, Korea, the Philippines and Vietnam there are certain full moon Uposatha days of importance. First Full Moon Festival, which is celebrated in Buddhist temples and also acknowledges the end of the Lunar New Year. Buddha's Birthday/Vesak

