Depending on the culture and time period, uposatha days have been observed from two to six days each lunar month.
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.
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.
In China, Japan, Korea, the Philippines and Vietnam there are certain full moon Uposatha days of importance.