What in-game date does this night fall on?

[Ravemore]

Holy Days

Sublimis

"There is only one day that is celebrated as a holy day in the Os Inasnum, and that day comes during the New Moon Cycle when both Ausir and Orn become new simultaneously and start their revolutions of light again. This night, legendary and feared by the rest of the world, is called Sublimis ("raised"). On this night, followers of Corath will overrun their local areas with armies of unliving; while the Sancti raise any living that falls before them and so feed the army's size. Like locusts, the horde will grow as it moves until daylight hits and they are driven back. Many loved ones are lost in this sheer display of power and evil. On this night, clerics of other gods will gather in towns and cities to form a divine wall against those who would defile the citizens. Farmers and people living in the countryside or outskirts will seal their homes and travel to the city for protection..."

[cbnicholson]

This looks familiar to the Rofirein celebration every 28 days or Ausir Shining or I guess at its known now as Vigil of the Dragon's eye. There's no set date that I'm aware of to these cycles other than every 28 days.

[Dorganath]

The moons, Orn and Ausir, don't necessarily follow a 28-day pattern like they do on Earth. In fact, I'm pretty sure they don't, though I also do not know the correct answer.  If memory serves, there's something non-standard about their orbits, possibly a magical influence.

In fact, if I remember right, there's a conjunction of the two moons that is called The Red Moon which only happens once every 300 years.