The night in which the moon, in conjunction with the sun, is invisible from the earth and the full moon night played an important role in Ancient India. It forms a basis of Vedic rituals to stay these two nights around the sacred fires with religious observance, above all fasting, and to perform the next morning the New and Full Moon Sacrifices. The Upavasatha originally means ‘staying overnight around the sacred firesʼ, so that it should have started from sunset. In the later ritual system, however, it was extended to the whole ritual procedures in the preceding day of the New and Full Moon Sacrifices, thus from sunrise.
About the 5 century B.C. it seems to have been deeply rooted in the life of people to stay the new and full moon nights (as well as a waxing and waning half-moon nights) with control of instinctive desires, and thus it was adopted even in Buddhism and Jainism which denied Vedic rituals: Pāli uposatha-/posatha-, BHS upoṣadha-/poṣadha-, Amg posaha-. The Uposatha for the Buddhist monks and nuns is a ceremony of reciting the precepts (Pāli Pāṭimokkha/Pātimokkha, BHS Prātimokṣa) in an assembly at the new and full moon nights and seems to have been inherited from the Vedic Upavasatha. For the Buddhist laypeople, on the other hand, the Uposatha takes place on the 8th, 14th, 15th days of a half month. On these days, from early morning till the next morning, 8 constituents of Uposatha (8 moral behaviors) are practiced, among which special importance was laid on fasting, as is seen in the prose narratives of the Jātaka.