666 Years
So we have the 666 years - every time this happens apparently a certain conjunction happens that puts a magical spotlight on the earth. Any human born in that light has the potential to wield power over the Outsiders. Whether there is a ritual (i'd guess a sort of baptism) required to prepare or lock in the power. The fact it happened on Halloween also seems significant - considering that's one of the few conjunctions that makes immortals mortal (when on earth).
Ebeneezer doesn't explain all the rules but he does say it means Harry can touch them and banish them without being tainted. He is immune. He can even physically hit them and it *somehow* will still actually hurt the being - even if they normally can't feel pain. I would say he even might be able to kill them. I suspect it's related to thaumaturgy (something Harry has always been good at) and the intention that comes with magic. Free will and all that.
But Ebeneezer says the event only lasts a few hours, and it's already happened (hence Harry). Which is interesting considering the previous time it happened, the Black Plague showed up (also when Nicodemus and Tessa last worked together prior to the series).
The Cycle
What this tells us is that following the events of the 666 years conjunction, another event immediately follows. I don't know how many years it takes but Harry was born on 31st of October 1975 (give or take a year or two). So 1975 - 666 gives us the year 1309. This is approximately the birth of the last Starborn. Current candidates include Nicodemus, Mavra, Dracula (not Drakul) and potentially Kincaid. Ancient Mai is actually too young sadly. Funnily enough, potentially Bob (depending on how old you think he is...it's inconsistent) and I know he is a spirit but hey, maybe he came from the head of a starborn.
Anyway, the Black Plague kicks off in Europe around 1346. So 37 odd years later something should happen right?
Wrong. Because the actual timeline of the plague (at least historically) is much older. It goes like this.
3500-3000BC: Neolithic Decline. The plague is potentially the cause of this event.
541-750BC: The "First" Plague pandemic spreads from Egypt to the Mediterranean. Rome suffers the Plague of Justinian (considered to be the same plague). The curiously may also tie into the deaths of the first born in Egypt.
1346-1840: The "Second" plague pandemic may have spread from China due to Mongol destruction. The Black Death kicks off in 1346 to 1353 as is considered the worst pandemic in human history. The fire of London seems to end it the worst of it...but it continues to kill about a billion people from 1347-1665.
1866-1960s: The "Third" plague pandemic (may also have originated in China), results in 2 million deaths there, and then 20 million in India. In the 1940s antibiotics drugs are developed which dramatically reduce the death rate.
It continues until the current day but mostly people only die due to lack of treatment.
So because of the broad stretches of history that the various plagues cover we have to look at the big events.
1. The proto-plague that resulted in Neolithic decline ~3316
2. Plague of Justinian ~652
3. Black Death 1346
4. The Second Plague ~1866
But also around 0-5BC Jesus is born. Funnily enough, under a star....He is crucified approximately 33AD. A big event in the Dresdenverse where apparently many balances of power were reset and the Denarians and the Knights first show up. Possibly Nicodemus also running around. This also plays into the Cycle.
So it seems to me that a final conjunction must occur, a planetary alignment that allows for the re-balancing of the board. I suspect, just like Halloween, it's the only time where things can really change. Hence why everyone is so eager to take advantage of it. However, I also suspect that it also allows the Outsiders to gain some advantage and hence it is also an extremely destructive time. Which is also why so many of those who know are grabbing as much power as they can to defend themselves.
The question then is why hasn't whatever is going to happen yet, happened?
My WAG is that because we don't always have an exact rotation around the Sun or even on our own axis, the math is out. So almost no one is *exactly* sure of when this conjunction will happen but they know it's any day now and are getting their ducks in a row.