Let me add some more info as comment and ask this question once more.

"Why the full Moon days in Thailand (for important Buddhist occasions) are NOT the same as the full Moon say as observed/predicted by observatories around the world?"

Some questions and answers:

http://resources.woodlands-junior.kent.sch.uk/time... :
People in different hemispheres see the moon in a slightly different way.
In the Southern Hemisphere, people see the moon 'upside down' so the side which is shining (sunlit) seems the opposite from the Northern Hemisphere.

Countries in the different hemispheres see the Moon from a completely different vantage point from each other.

In the northern hemisphere the first quarter looks like a growing D, while in the southern hemisphere it looks like a C.

In the northern hemisphere the last quarter looks like a C, while in the southern hemisphere looks like a D.

[To memorise N-DOC = Northern hemisphere - first quarter looks like D, fullmoon like O, last quarter like C; and S-COD = Southern hemishere - first quarter looks like C, full moon like O, last quarter like D]

"Do you see different phases of the Moon around the world?"
Asked by Jonathan O'Callaghan, 13 December 2012

"Does the Moon look different from the northern and southern hemispheres?"
Asked by Brian Baur

The phases of the Moon that we see are caused by the relative positions of the Sun, Moon and Earth. The phase of the Moon is defined by the proportion of the Moon lit up by the Sun that is visible from Earth. Over the 24 hour period that it takes for the Earth to spin so that all areas can see the Moon, these relative positions wouldn’t alter enough to see a different phase of the Moon around the world.

[My Comment: The Moon phases are "visually" the same around the world (but at different "World Time"). That is the Moon looks full in a Thailand night also looks full in Canada or USA but the local time will be some 9 hours later than Thailand time.]

However the Moon does not look completely identical from every location on Earth; depending how far South or North you are (your latitudinal position) the Moon appears to be rotated. In the northern hemisphere the sunlit part of the Moon travels from right to left while from the southern hemisphere the light appears to travel from left to right. This is simply down to the differing angles you are observing the Moon from.

Answered by Megan Whewell, Education Team Presenter for the National Space Centre

Now another question: Who (or what oraganizarion) in Thailand determines what day is Visakha day,? Which day is Aasalaha day?... Why were they NOT the full moon days in 2558?