Moon days are when the moon is either New or Full. In Ashtanga yoga we do not practice on these days. It is considered that the energy is very strong at these times, sometimes even making us behave crazily. It is considered best to rest on moon days as it can be easier to injure ourselves at these strong energetic times.
If we consider that we are arounds 70% water, and what effect the moon has on the ocean, it would seem almost crazy not to think that we would also be effected by the moons monthly cycles.
The moon day can vary slightly depending on where in the world you are.
In Hindu tradition it is considered that the day starts with sunrise, so moondays in which the full or new moon falls between midnight and sunrise will be taken the day before.
upcoming moon days in Koh Phangan