Uposatha in detail
"And what is the Uposatha of the Noble Ones? It is the cleansing of the defiled mind through the proper technique. And how is the defiled mind cleansed through the proper technique?
"There is the case where the disciple of the noble ones recollects the Tathagata, thus: ‘Indeed, the Blessed One is worthy and rightly self-awakened, consummate in knowledge & conduct, well-gone, an expert with regard to the world, unexcelled as a trainer for those people fit to be tamed, the Teacher of divine & human beings, awakened, blessed.’
As he is recollecting the Tathagata, his mind is calmed, and joy arises; the defilements of his mind are abandoned, .
To Mahanama
Mahanama, at any time when a disciple of the noble ones is keeping in mind the Tathāgata, his mind is not overcome with passion, not overcome with hate, not overcome with delusion. His mind is straightened, based on the Tathāgata.
And when the mind is straightened, the disciple of the noble ones gets inspired by the goal, gets inspired by the dhamma, gains joy connected with the dhamma.
When he is joyful, rapture arises.
When the heart is rapturous, the body becomes tranquil. When the body is tranquil, he experiences happiness.
When he is happy, the mind becomes composed.
Mahanama, you should develop this recollection of the Buddha while walking, standing, sitting, or lying down. You should develop it while engaged in work and while living at home in a house full of children.
Furthermore
"There is the case where the disciple of the noble ones recollects the Dhamma, thus: ‘The Dhamma is well-expounded by the Blessed One, to be seen here & now, timeless, inviting verification, pertinent, to be realized by the wise for themselves.’ As he is recollecting the Dhamma, his mind is calmed, and joy arises;
…
"There is the case where the disciple of the noble ones recollects the Sangha, thus: ‘The Sangha of the Blessed One’s disciples who have practiced well… who have practiced straight-forwardly… who have practiced methodically… who have practiced masterfully — in other words, the four types [of noble disciples] when taken as pairs, the eight when taken as individual types — they are the Sangha of the Blessed One’s disciples: worthy of gifts, worthy of hospitality, worthy of offerings, worthy of respect, the incomparable field of merit for the world.’ As he is recollecting the Sangha, his mind is calmed, and joy arises;
Details;
Monks, those monks who are accomplished in virtue, accomplished in meditative composure, accomplished in knowing ability, accomplished in liberation, accomplished in the knowledge and vision of liberation: even the sight of those monks is helpful, I say; even listening to them… even approaching them… even attending on them… even recollecting them… even going forth after them is helpful, I say. For what reason? Because when one has heard the Dhamma from such monks one dwells secluded by way of two kinds of seclusion — seclusion of body and seclusion of mind.
Abiding thus secluded, one keeps in mind that Dhamma and thinks it over. Whenever, monks, a monk abiding thus secluded keeps in mind that Dhamma and thinks it over, on that occasion retention as a factor of enlightenment is aroused by a monk, on that occasion a monk develops retention as a factor of enlightenment, on that occasion retention as a factor of enlightenment comes to fulfilment through development in a monk.
(Other factors of enlightenment become fulfilled as well)
Furthermore
"There is the case where the disciple of the noble ones recollects his own virtues, thus: ‘[They are] untorn, unbroken, unspotted, unsplattered, liberating, praised by the wise, untarnished, conducive to concentration.’ As he is recollecting virtue, his mind is calmed, and joy arises;
An example;
Approaching them [monks], I will ask, ‘Venerable sirs, did the master [Buddha] ever come to Sāvatthī?
If they tell me that he did come to Sāvatthī, then I will conclude that ‘Undoubtedly the master used a rains-bathing cloth of mine, or a meal for newcomers, or a meal for those going away, or a meal for the sick, or a meal for those tending the sick, or medicine for the sick, or gruel gifted every day.’
For me, recollecting that, joy will be born. When joyful, rapture will be born. When the heart is rapturous, the body becomes tranquil. When the body is tranquil, I will experience happiness. When I am happy, the mind will become composed.
That will be the development of my (five) faculties, (five) strengths, and (seven) factors of enlightenment.
Furthermore
There is the case where the disciple of the noble ones recollects the devas, thus: ‘There are the Devas of the Four Great Kings, the Devas of the Thirty-three, the Yama Devas, the Contented Devas, the devas who delight in creation, the devas who have power over the creations of others, the devas of Brahma’s retinue, the devas beyond them. Whatever conviction they were endowed with that — when falling away from this life — they re-arose there, the same sort of conviction is present in me as well. Whatever virtue they were endowed with that — when falling away from this life — they re-arose there, the same sort of virtue is present in me as well. Whatever learning they were endowed with that — when falling away from this life — they re-arose there, the same sort of learning is present in me as well. Whatever generosity they were endowed with that — when falling away from this life — they re-arose there, the same sort of generosity is present in me as well. Whatever discernment they were endowed with that — when falling away from this life — they re-arose there, the same sort of discernment is present in me as well.’ As he is recollecting the devas, his mind is calmed, and joy arises;
Furthermore one emulates the Arahants, these are the training precepts;
Furthermore, the disciple of the noble ones reflects thus: 'As long as they live, the arahants — abandoning the taking of life — abstain from the taking of life. They dwell with their rod laid down, their knife laid down, scrupulous, merciful, compassionate for the welfare of all living beings.
"'As long as they live, the arahants — abandoning the taking of what is not given — abstain from taking what is not given. They take only what is given, accept only what is given, live not by stealing but by means of a self that has become pure.
"'As long as they live, the arahants — abandoning uncelibacy — live a celibate life, aloof, refraining from the sexual act that is the villager’s way.
"'As long as they live, the arahants — abandoning false speech — abstain from false speech. They speak the truth, hold to the truth, are firm, reliable, no deceivers of the world.
"'As long as they live, the arahants — abandoning fermented & distilled liquors that cause heedlessness — abstain from fermented & distilled liquors that cause heedlessness.
"'As long as they live, the arahants live on one meal a day, abstaining from food at night, refraining from food at the wrong time of day [from noon until dawn].
"'As long as they live, the arahants abstain from dancing, singing, music, watching shows, wearing garlands, beautifying themselves with perfumes & cosmetics.
"'As long as they live, the arahants — abandoning high & imposing seats & beds — abstain from high & imposing seats & beds. They make low beds, on a pallet or a spread of straw.
Today I too, for this day & night — [will be like this or that] By means of this factor I emulate the arahants, and my Uposatha will be observed.
As to why one shouldn’t use imposing seats & beds, i think it is because the Arahants are modest, content with little and their comfort is derived from a mind well tamed & freed from obsessions, not the comforts of imposing beds.
“Always, always,
he sleeps in ease:
the brahman totally unbound,
who doesn’t adhere
to sensual pleasures,
who’s without acquisitions
& cooled.
Having cut all ties
& subdued fear in the heart,
calmed,
he sleeps in ease,
having reached peace
of awareness.”