Full Moon Declaration Events
Shapeshifting and magical disguises are much more difficult to maintain during the full moon. It is This is well known among the upper classes but this is not universally, especially among the lower classes. In other words, suspicious farmers are not going to scrutinize their neighbors under a full moon but those in the halls of power often want to take precautions against shapeshifting spies, especially Metamorphs. A full moon declaration is essentially a formal meeting that occurs during a full moon, usually outside for at least part of the meeting. It's not a giant faux passe to skip attendance at a full moon declaration, but skipping two will raise a lot of eyebrows and skipping three will create overt submission. In this way, no metamorph can realistically maintain a high level impersonation for more than a season.
The full moon has been used to shine light on those wearing false faces since the dawn of the First Age, but formal full moon declarations are very much a cultural artifact of the Third Age. Metamorphs were never particularly trusted or liked by the other mortal races in the Second Age, but prejudice and suspicion against them was magnified during the Second Unmaking when many Metamorphs became Infernalists. Tales of the betrayal of the Metamorphs spread into the Red Era of the Third Age creating (possibly justified) paranoia which led to the evolution of full moon declarations.
Sometimes, full moon declarations are just brief meetings where people state their names and reaffirm their loyalty to their liege, but other times full moon declarations are more elaborate. To take the sting out of what is, in essence, a paranoid inquisition, many lords and ladies will provide entertainments: night time banquets, musical performaces or even plays (though some superstitious actors refuse to wear a compelx costume on a full moon). Since the full moon represents Zarthus' gaze, and Zarthus believes in holding rulers accountable for their actions, the nightly entertainment might involve jesters or others making jibes at the local lord or ladie's expense. Other rulers allow subjects to (respectfully) air grievances or complaints during the full moon in a more direct and less playful way.
Some high level lords and ladies make their high level retainers formally present themselves during the full moon. More paranoid rulers make their entire household staffs present themselves during the full moon.
Some lords and ladies host a full moon declaration every lunar month. Some choose to do so every other month. A few host very large full moon declarations once or twice a year. Sometimes these larger events are folded into Zarthus worship rituals. On a less regular basis, high level negotiations and treaty signings between different nations often schedule these events on the night of the full moon when possible.