According to the boss of a Gold Dungeon, the Aether is kept in perpetual day due to its influence. However, there are periods of time when the world suddenly darkens about 4 light levels over the course of a second or two. This makes it slightly easier for the hostile Cockatrices to spawn, but they are still confined to underneath overhangs. Sometimes, it will get even darker, as shown in the second picture on the right. This makes it seem like "2 nights overlapped.", which happens when there is rain and/or thunder in the overworld. The sky darkening effect in the overworld during rain or thunder is the same in the aether, but without the rain.However, after the boss of any gold dungeon is killed, the day and night cycle will precede, remaining so permanently on that world.
It appears that day and night are not of a set length. There have been instances where nighttime lasted less than a minute, or longer than fifteen.
According to the Pro tips that pop up when a world is loading, day in the Aether lasts 3 times longer than day on the surface.
If a gold boss has been killed and it is night, then rain starts in the Overworld followed by thunder, it can look like "3 nights overlapped".