Blackmoon

Although the stoneworks front of the Mercenary's Guild was only a facade facing the street, the building itself was tall enough so that the stone wall served as a rooftop parapet. Dusk had just fallen, and the city was unusually hushed for such an early hour, though the rooftop power-shafts kept clacking and rotating in the dark.
Vance pointed out the canal gondola rope tower, a silhouette only barely visible past the street candles. "That's easily the best spot in this whole city for Blackmoon, but they won't let anyone into the tower after sunset. So this is the second best place to watch."
"Sounds impressive, but if that's the case, why are we the only two up here? Or did you manage to convince them that this was going to be a special night for you?" Alais gave Vance an impish grin, twisting a silk sleeve. "You cad!"
"First off, it's because the door is locked normally, and I have the key, and secondly, Rathnait would strangle me in my sleep if she found out." Vance grimaced. "Besides, you're not my type, and I tried being a noblewoman's weekend fancy once. It didn't work out."
"No fun at all." The girl gave a pout, before glancing at the sky. She immediately stood and ran to the parapet, looking out over the street below. "It's starting!"
The moon had just the barest chip in its side. Tonight's eclipse was only meant to last just under an hour, but Blackmoon was observed without regard for duration, and the pyres had been built to the same size. Vance stepped next to Alais and watched as the bonfire just outside the Guild gate was ignited, to loud cheers. Fires across the city began to go up now, bathing the streets in flickering firelight, the power-shafts along the roofs now casting shifting shadows along the walls. 
Vance heard Alais cheer suddenly, and turned just in time to see a wooden horse leave her hand, thrown from the roof and dropping right in the middle of the bonfire. When Alais looked back at him, she stuck out her tongue.
"I'm going to tell Rathnait you did that." Vance uncorked a wine bottle and sat heavily against the parapet. "This is going to be a long night."

History

Blackmoon is a syncretic festival combining many of the regional traditions and practices surrounding the lunar eclipse, a semi-regular event across most of Saibh. Astronomers have been able to calculate the dates of lunar eclipses to precision since antiquity, and the day prior to the eclipse has been treated typically as a public holiday, with the pre-Unification kingdoms and localities each observing their own rites.
After The War of Unification, the First Princeps engaged in the largest social engineering project in history, attempting to unify all of the disparate cultures and peoples of the West Saibh continent into his new Principality of Etoile. Part of this process was establishing new cultural institutions that were uniquely Etoilean, instead of simply overlaying his own Nassean culture over the continent, and the lunar eclipse festival was one of two universal holidays that virtually every culture on the continent observed (the other being the solstices). 
Thus, the festival of 'Blackmoon' was institutionalized by the Principality as a state holiday, incorporating many of the facets of the largest festivals, while allowing each community and locale to add their own flairs based on their old cultural histories.

Execution

The Etoilean 'standard' form of Blackmoon is a fire festival. Each community that undergoes eclipse (not all places in Saibh are eclipsed on the same day) will spend the day preparing materials for the evening, by ancient custom, 'offerings', constructed papercrafts made in the image of valuable goods (such as 'gold bars' made from yellow paper, or piles of papercraft fruit). When the eclipse begins or when the sun sets (whichever comes later), a network of bonfires are lit in each town, each fire within direct line of sight of another. The community is tasked with keeping the fires burning brightly to ward off the darkness - every home must be illuminated by firelight throughout the duration of the eclipse.
This is typically not a serious concern, especially as modern city streets are already illuminated by public tallow lights, so most participants buy an offering during the day, throw it into the bonfire upon the eclipse, and spend the night dancing and drinking around the flame.
The festival is highly varied outside of its standardized form. Many communities incorporate some sort of costumed spectre that is tasked with stalking the dark parts of a town (mostly to scare children), to be ritually warded off by the bonfire flames. Others build single bonfires for the community, but build a pyre stack so high that the flames can be seen from anywhere in town, bright enough to drive off the night entirely.

Components and tools

Common to all festivals is the materials to be burned - pyres are constructed with oiled hardwood for a longer lasting burn, while local craftsmen spend a few days before the eclipse making small papercrafts and woodworks to be burned in the bonfire. Most important with festivals like these, as always, are the people, who gladly take the day off to burn off steam and enjoy the evening.

Participants

Blackmoon festivals are held in all Etoilean communities larger than a roadside rest stop, with virtually everyone in the Principality near enough to a bonfire to throw in their offering. Each individual bonfire is managed by a Flametender, a title that held ritual significance in the past, but in the modern era is simply the volunteer to make sure the fire doesn't die down over the night.

Observance

Lunar eclipses are forecast many years in advance, with their occurrence and location in Saibh being known. Blackmoon festivals are held during each and every lunar eclipse in the affected communites, no matter if they occur several years apart or only a few days apart (as has happened twice at the island-city of Nenuph within the last fifty years).