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Comm. NT: 10. Uposatha (der. from upavasati, to observe or to prepare) is the name for the day of “fasting” or “vigil” observed on the days of the new moon, waxing half moon, full moon, and waning half moon. On these days it is customary for laymen to undertake the Eight Precepts (sīla) or Five Precepts. On the new-moon and full-moon days the Pātimokkha (see note 11) is recited by bhikkhus. The two quarter-moon days are called the “eighth of the half moon.” The Full-moon day is called the “fifteenth” (i.e. fifteen days from the new moon) and is the last day of the lunar month. That of the new moon is called the “fourteenth” when it is the second and fourth new moon of the four- month season (i.e. fourteen days from the full moon), the other two are called the “fifteenth.” This compensates for the irregularities of the lunar period.

пали	Upāsakaupāsikānaṃ niccasīlavasena pañcasikkhāpadāni, sati vā ussāhe dasa, uposathaṅgavasena aṭṭhāti idaṃ gahaṭṭhasīlanti evaṃ bhikkhusīlādivasena catubbidhaṃ.
Nyanamoli thera	The five training precepts—ten when possible—as a permanent undertaking and eight as the factors of the Uposatha Day,10 for male and female lay followers are the virtue of the laity. So it is of four kinds as the virtue of bhikkhus, and so on.