Is Uposatha on full moons or specific days?

"Which day is Uposatha?" is a good question and very importand for Bhikkhus who even need to ask if not knowing. Since it is not easy to answer, best keep the Uposatha always, like Upasaka Lanka already mentioned in his answer.

Actually, althought the Buddha allowed his monks to learn the calculation of the full moon and half moon days, there seems to be no (public/broadly know) transmission.

Since the day depends on the certain location (note that the day starts at sunrise not at 00:00! to understand, which can be very different), althought the event of full or empty is equal for the whole world, is best to request or inform you from the Bhikkhu-Sangha where you took/asked/declared refuge and received the precepts or you follow, if not avaliable, your near personal community.

The simple counting, starting from the full moon day, is eight and then fourteen/fifteen days. So it matches 29 Days a month. Since a moon-month has a little bit more than 29 days, different traditions compensate it in different ways.

As for the traditional observing and gathering days (note that the more often or even always, the better: To the Sakyans (on the Uposatha)) they are held "weekly" (see Upasaka Suminda Sirinath S. Dharmasena answer) at large but also on further days:

pakati-uposatha pati-jagara-uposatha pati-harika-pakkha-uposatha

The times for keeping the pakati-uposatha are the 5th, 8th, 14th and 15th of the waxing moon and the 5th, 8th, 14th and 15th of the waning moon.

The times for keeping the pati-jagara-uposatha are the five days of the waxing moon, i.e., the 4th, 6th, 7th, 9th, and the 13th, and the six days of the waning moon: the 1st, 4th, 6th, 7th, 9th, and 12th or 13th. That makes eleven days in a month for the observance of this type of Uposatha.

The four months of the rainy season, or vasso, starting on the first night of the waning moon of the seventh month and ending in the middle of the eleventh month, is the period for observing the pati-harika-pakkha-uposatha.

The commentary to the Raja Sutta explains that the pati-harika-pakkha-uposatha is the Uposatha that is observed continuously throughout the three months of the rains. If one cannot keep the observance for the full three months of the rains, then it should be kept for one month, from the first day of the waning moon of the tenth month to the eleventh month. If one is not able to keep the observance for one full month, then it should be kept for a half month, from the first day of the waning moon of the tenth month to the end of that month. Any period of this observance is called pati-harika-pakkha-uposatha.

Some texts define (the time for keeping the pati-harika-pakkha-uposatha ) as the five months from the seventh month up to the eleventh month.

Some Acariyas say the three months are the seventh, eleventh and third months.

Yet other sources explain that the four days, i.e., the 7th, 9th, 13th and 14th, both waxing and waning, are the only days for the observance of the pati-harika-pakkha-uposatha.

(source)

In regard of which precepts are observes, the questions list misses abstaining beautifications and participation on entertainments. Upasaka Lanka had listed them all well in his answer. But next to the many commentaries and explainings deriving from monks Vinaya, the Sutta in its regard might be the best: Uposatha Sutta: The Uposatha Observance.

Generally possible useful help on how might be found here: How to come to be able to observe precepts?

There might be found answers to other details when searching for Uposatha here. How ever, best to speak direct and gain advices from those successfull practice them (of which are maybe less and not always to find).

In regard of the detail question "does this need to be vegetarian to fit in with the first precept?": Not unconditionally. Importand not to kill for yourself, bring others by words or signs to kill and not to feel joy or acceptance, mental approve, for killing. The last includes not to eat meat if a living being was killed for you personally (if seen, heard or suspected). Eating what ever is not alive, does not carry any unskillfulness within it. Killing, urge to or agree, does and whould for the bodily and verbal action cause a break of the first precept, and bad effects (kamma) in all three ways of actions.

May you put effort on the foundation for liberation and observe the Uposatha eager, and if not done or forgotten, todays event, now, is for sure the best time to start or to re-start.

Sīlena sugatiṃ yanti. Sīlena bhoga-sampadā. Sīlena nibbutiṃ yanti. Tasmā sīlaṃ visodhaye.

Through virtue they gain a good stream. Through virtue they gain wealth. Through virtue they come to liberation. Therefore we should clean our virtue.

Anumodana

[Note: This is a gift of Dhamma and not meant for commercial purpose or other low wordily gains by means of trade and exchange.]

You said 8 and listed 7 -- I think you're missing one.

The one you didn't mention is the one about "flower garlands and music" which I understand to mean "pursuing sensuality".

I think people are meant to give that (pursuit of sensual delight) a rest, and instead have some time to think (i.e. meditate).

Buddhists practice eight precepts to learn buddha's teaching and practice samatha-vipassana at forest monastery.

Nowadays, most of upasaka can not go to forest monastery at uposatha-days, such as me, so I think "day-off" is the best day to done eight precepts. On day-off, we can learn and practice samatha-vipassana, without any other jobs.

Uposatha-day is the day that people make an appointment together to do eight precepts, learn, and practice samatha-vipassana. So the date is up to your appointment with your teachers and practicer-friends group.

It is look like sunday, that is day off of christian to go to church to learn with pastors.

This is thai moon calendar

https://www.myhora.com/%E0%B8%9B%E0%B8%8F%E0%B8%B4%E0%B8%97%E0%B8%B4%E0%B8%99/

If you want to find out the traditional days you can use a moon phase website. E.g. https://www.timeanddate.com has moon phases. For Waterloo following are the moon phases: https://www.timeanddate.com/moon/phases/usa/waterloo. For other US cities you can browse USA: https://www.timeanddate.com/moon/phases/usa/.

For the purpose of Uposatha the day is counted from sunrise to sunrise so start at sunrise and keep it up until the following day's sunrise if possible.

The general days are:

• the new moon,
• the full moon, and
• the two quarter moons in between

or in Burma:

• waxing moon,
• full moon,
• waning moon, and
• new moon

Since generally you are free on weekends or other holidays you can use these to practice. You do not have to be strict on the days.

Similarly the time you can have food is between sunrise and solar noon. You can find the sun rise and and the solar noon: https://www.timeanddate.com/sun/usa/waterloo

Regarding eating it is advisable to be strict to keep to the precepts. General rule of thumb of breakfast at 6:30 and lunch at 11 will keep within the normal sunrise and solar noon times, unless there are daylight saving adjustments or in time zones where there is disparity the solar position.

I want to start observing Uposatha, but I'm a little unclear on what days it is. I'm finding calendars that it is the days of full moons in Sri Lanka or other countries. Should it just be celebrated on full moons then?

You can use the calendars from e.g. Sri Lanka, Thailand, Myanmar etc.

More importantly is the practice itself, ie. the deepening of ones practice on special days of the months. Its really that which is essential from a practical Dhamma perspective.

The Wikipedia describes it as;

The Buddha taught that the Uposatha day is for "the cleansing of the defiled mind," resulting in inner calm and joy. On this day, both lay and ordained members of the sangha intensify their practice, deepen their knowledge and express communal commitment through millennia-old acts of lay-monastic reciprocity. On these days, the lay followers make a conscious effort to keep the Five Precepts or (as the tradition suggests) the Eight Precepts. It is a day for practicing the Buddha's teachings and meditation.

You can use full moon days in your country or you can use the traditional calculated calendar days.

Also, am I correct that the eight precepts to be practiced on Uposatha are. Are these correct?

The Eight Precepts are:

1. Panatipata veramani sikkhapadam samadiyamiI undertake the precept to refrain from destroying living creatures.

2. Adinnadana veramani sikkhapadam samadiyamiI undertake the precept to refrain from taking that which is not given.

3. Abrahmacariya veramani sikkhapadam samadiyamiI undertake the precept to refrain from sexual activity.

4. Musavada veramani sikkhapadam samadiyamiI undertake the precept to refrain from incorrect speech.

5. Suramerayamajja pamadatthana veramani sikkhapadam samadiyamiI undertake the precept to refrain from intoxicating drinks and drugs which lead to carelessness.

6. Vikalabhojana veramani sikkhapadam samadiyamiI undertake the precept to refrain from eating at the forbidden time (i.e., after noon).

7. Nacca-gita-vadita-visukkadassana mala-gandha-vilepana-dharana-mandana-vibhusanathana veramani sikkhapadam samadiyamiI undertake the precept to refrain from dancing, singing, music, going to see entertainments, wearing garlands, using perfumes, and beautifying the body with cosmetics.

8. Uccasayana-mahasayana veramani sikkhapadam samadiyamiI undertake the precept to refrain from lying on a high or luxurious sleeping place.

In the comments you asked about the 1st precept;

And should the meal before noon be vegetarian to coincide with the first precept?

Several factors must be fulfilled in order to break the 1st precept. The five factors of the first precept are:

• pano -- presence of a living being.
• panasannita -- one knows that it is a living being.
• vadhacittam -- the intention to kill.
• upakkamo -- the effort to kill.
• tena maranam -- the resulting death of that being.

If one of these factors are not fulfilled, the precept is still intact.

For more information, please see "Ask A Monk: Mark Zuckerberg, Vegetarianism and Killing" by Ven. Yuttadhammo.