The Uposatha (Sanskrit: Upavasatha) is Buddhist day of observance, in existence from the Buddha's time (500 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, lay disciples and monks intensify their practice, deepen their knowledge and express communal commitment through millennia-old acts of lay-monastic reciprocity.

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 phases of the moon: 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 Burma, Uposatha (called ဥပုသ် 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 pre-colonial Burma, the Uposatha day (commonly rendered as the Buddhist Sabbath in English) 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 the Uposatha day as the legal day of rest. All major Burmese Buddhist holidays occur on Uposatha days, namely Thingyan, the beginning of the Buddhist lent (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.