Uposatha: lit. 'fasting', i.e. 'fasting day', is the full-moon day, the new-moon day, and the two days of the first and last moon quarters. On full-moon and new-moon days, the Disciplinary Code, the Patimokkha is read before the assembled community of Bhikkhus while on the mentioned 4 moon-days many of the faithful lay devotees go to visit the monasteries, and there take upon themselves the observance of the 8 precepts attha-sīla; sikkhāpada See A. VIII, 41ff.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.