The meaning of uposatha and its outset
Firstly the real sense of uposatha is to be understood. In connection with the meaning of it, it is somewhat at variance with individual likings. Thanissaro bhikkhu articulates that the word uposatha derives from Sanskrit upavasatha, a day of preparation, usually connecting particular observances, for a formal procedure. I.B Horner states that uposatha means a formal act of observance when Melford E. Spiro defines it the collective recitation of the rules whereas G.S.P. Misra explicates that uposatha is clarified as the most unifying factor. Somdet Phra Mahæ Sama¼a Chao, one of the Thai scholar monks also interprets the word uposatha as entering to stay. The definition of uposatha according to grammatical point of view, however, identifies it living in purity.
Canonical texts explain that wandering ascetics held assemblies on 14th or 15th ansd 8th days of the fortnight to deliver their Dhamma (teachings) to the lay devotees who came to attend it. The Buddha adopted this custom when King Bimbisæra requested to do so. The Buddha allowed monks to meet in unison and teach the Dhamma on those days. In later period the Buddha launched a purely monastic uposatha performance only twice a month limited to the final day of the lunar fortnight and thereafter formulated that the recital of pætimokkha, what he laid down for the monks, was to be used as a formal act of uposatha performance. Naturally the Buddhas with longer life-expectancy never promulgate rules and regulation for the monks and bhikkhunøs and not allow uposatha performance to the monks due to the state of purity of the order.
What the longer-life-span-Buddhas generally do is to admonish monks occasionally with three verses starting with Khantø paramaµ tapo titikkhæ (forbearing patience is the noble practice).
The Buddhas whose life spans are short, nonetheless, always lay down disciplinary rules for their order and uposatha practice is allowed as well. The Gotama Buddha had a shorter life span, therefore he laid down the rules for monks and bhikkhunøs and made allowance of uposatha observance to the monks. He practised ovædapætimokkha amidst monks every full and new moon day, but owing to that an impure monk was permitted to participate in the ceremony of ovædapætimokkha, the Buddha stopped practising it within his first period of twenty years after the achievement of enlightenment (pathamabodhi). Some years from this event the Buddha permitted the recital of pætimokkha for uposatha performance.