Blue Moons

Once in about every nineteen years, there is a 13th full moon in the Nirian calendar, making a 13th month, the Month of Betrayals.
The name comes from an alternate name for the blue moon: the Betrayer Moon.
Historically, the Betrayer Moon got it's name as being the last remnant of Midur's betrayal of the Elemental Godx. After Midur betrayed the Elementals' plans to Aher in the hopes that the conflict would be settled peacefully prematurely, Ramos was so enraged that he conspired with Ysthos to make a "month of sour luck. Of betrayal, to dash the naive hopefulness of Midur's ilk against the rocks of time."
It's said that people born in the Month of Betrayals will either be chronically unlucky, or are inherently evil and should not be trusted.
Werefolk are particularly affected by Betrayer Moons and tend to shift every night for one or two nights before the moon reaches true fullness -- some are even unlucky enough to shift every night a full moon phase ahead.