Hmmmm.
Well, you’re talking about episode in Season 3 called The Southern Raiders which does take place during a Full Moon, hence Katara’s ability to cause that huge storm.
As for if it’s day or night, that would seem irrelevant since “the full moon” is a time period, not necessarily a visual. There can be full moons in the daylight, we just can’t always see them because not enough light is reflecting off the lunar surface. There are even places on Earth that don’t get full moons every month.
And – not to get super picky – but we don’t even know if this world is even close to Earth, so really all these “moon rules” are probably screwy.
-Da7e-