At the time of the rise of Buddhism the word had come to mean the day preceding four stages of the moon’s waxing and waning, viz. 1st, 8th, 15th 23d nights of the lunar month that is to say, a weekly sacred day, a Sabbath. These days were utilized by the pre-Buddhistic reforming communities for the expounding of their views, Vin.i.101. The Buddhists adopted this practice and on the 15th day of the half-month held a chapter of the Order to expound their dhamma, ib. 102 They also utilized one or other of these Up. days for the recitation of the Pāṭimokkha (pāṭimokkhuddesa), ibid. On Up. days laymen take upon themselves the Up. vows that is to say, the eight Sīlas, during the day. See Sīla The day in the middle of the month is called cātudassiko or paṇṇarasiko according as the month is shorter or longer The reckoning is not by the month (māsa), but by the half-month (pakkha), so the twenty-third day is simply aṭṭhamī, the same as the eighth day. There is an accasional Up. called sāmaggi-uposatho, “reconciliation-Up.”, which is held when a quarrel among the fraternity has been made up, the gen. confession forming as it were a seal to the reconciliation (Vin.v.123; Mah. 42)
■ Vin.i.111 Vin.i.112, Vin.i.175, Vin.i.177; Vin.ii.5, Vin.ii.32, Vin.ii.204, Vin.ii.276; Vin.iii.164, Vin.iii.169; DN.iii.60, DN.iii.61, DN.iii.145, DN.iii.147; AN.i.205 sq. (3 uposathas: gopālaka˚ nigaṇṭha˚, ariya˚), AN.i.208 (dhamm˚), AN.i.211 (devatā˚); AN.iv.248 (aṭṭhanga-samannāgata), AN.iv.258 sq. (id.), AN.iv.276, AN.iv.388 (navah angehi upavuttha); AN.v.83; Snp.153 (pannaraso u); Vb.422; Vism.227 (˚sutta = AN.i.206 sq.); Sdhp.439; DN-a.i.139; Snp-a.199; Vv-a.71, Vv-a.109; Pv-a.66, Pv-a.201
■ The hall or chapel in the monastery in which the Pāṭimokkha is recited is called uposathaggaṃ (Vin.iii.66), or -āgāraṃ (Vin.i.107; Dhp-a.ii.49). The Up. service is called -kamma (Vin.i.102; Vin.v.142; Ja.i.232; Ja.iii.342, Ja.iii.444; Dhp-a.i.205) uposathaṃ karoti to hold the Up. service (Vin.i.107 Vin.i.175, Vin.i.177; Ja.i.425). Keeping the Sabbath (by laymen is called uposathaṃ upavasati (AN.i.142, AN.i.144, AN.i.205, AN.i.208 AN.iv.248; see upavasati), or uposathavāsaṃ vasati (verse 177) The ceremony of a layman taking upon himself the eight sīlas is called uposathaṃ samādiyati (see sīlaṃ & samādiyati); uposatha-sīla observance of the Up. (Vv-a.71) The Up. day or Sabbath is also called uposatha-divasa (Ja.iii.52).

Vedic upavasatha, the eve of the Soma sacrifice, day of preparation