Uposatha days are times of renewed dedication to Dhamma practice, observed by lay followers and monastics throughout the world of Theravada Buddhism.
(source: Access to insight)

Observance Day (Uposatha) is determined by the lunar calendar, falling on the full-moon day, the new-moon day, and the two quarter-moon days of each lunar month. The full-moon day is considered to be the most important of all of them. On the Uposatha day the monks (bhikkhus) and nuns (bhikkhunis) living in a community of four or more, gather to recite the rules of the monastic order (Patimokkha). In Buddhist countries lay Buddhists generally go to the Temple to pass the day and night, often observing the three refuges and eight precepts. They spend their time reading Dhamma books, listening to Dhamma sermons, meditating and discussing the Dhamma. Sometimes a lay Upasaka (man observing the precepts) or Upasika (woman observing the precepts) who is well versed in the Dhamma will preach Dhamma to the rest of the people if a monk or nun is not available.
(source: Buddhanet.net)