Ebon Noon
This holiday is observed by necromancers, warlocks, blackguards, and other practitioners of evil magic. It occurs only once every few years at most (rather than annually) and is dependent on the alignment of the sun and moon(s) in an eclipse of surpassing darkness. At the culmination of the eclipse all evil spellcasting is greatly enhanced for a few moments (caster level +5), and with the proper timing animate dead, divination, and other spells can yield great rewards.

This holiday is not common knowledge; it is generally observed only by rogues, assassins, shadowdancers, and all others who skulk and lurk in the darkness. A night when the criminal underworld forswears bloodshed amongst each other and with the common citizenry, and when burglars choose to share a flask of brandy on the rooftops rather than break in through a skylight. City guards don't know why crime is greatly reduced on this particular night of the year, but they're grateful for it nonetheless.

Opal Moon
This holiday is the direct inverse of Ebon Noon - a celestial alignment such that the radiant moon turns the night almost as bright as day for a few moments. The powers of good magic are greatly enhanced by the effect - lycanthropy, mummy rot, and similar pernicious curses can be removed with even the simplest spells at this time (or are perhaps instantly broken by the moonlight itself), and undead are harmed by the moonlight as though it were sunlight. Clerics, paladins, and other good-aligned divine spellcasters set aside their traditional suspicion of good-aligned arcane spellcasters for the night.